NOVELTY - A flow direction change period operation reaction device, comprises: (I) reactor (19) comprising packed bed catalytic reactor (5), two-position four-way valve, two two-way stop valves and connecting pipeline; and (II) heat exchange device comprising raw material pre-heating heat exchanger (9) and recovery heat exchanger (10). The packed bed catalytic reactor is divided into three layers, upper layer is heat exchange area used for filling an inert packing or having a solid catalyst; intermediate layer filled with solid catalyst; and lower layer is heat exchange area (6).    USE - As flow direction change period operation reaction device for methanation of synthesis gas (claimed).    ADVANTAGE - The device and the method prolong the service life of the catalyst and have high conversion efficiency. By changing feeding flow direction periodically, using the methanation catalytic reactor and heat exchanger integrated together, increases the integration degree of the process and improves the efficiency in the traditional non-computer method and improves selectivity, reduces catalyst deactivation.    DETAILED DESCRIPTION - A flow direction change period operation reaction device, comprises: (I) reactor unit (19) comprising a packed bed catalytic reactor (5), a two-position four-way valve, two two-way stop valves and a connecting pipeline; the packed bed catalytic reactor is divided into three layers, the upper layer is a heat exchange area (9) used for filling an inert packing or having a solid catalyst with reaction activity; the intermediate layer is a reaction area filled with a solid catalyst with reaction activity; the lower layer is a heat exchange area (6) used for filling an inert packing or having a solid catalyst with reaction activity; the upper end and the lower end of the packed bed catalytic reactor are respectively provided with openings for feeding and discharging the gas; a port (h) of the two-position four-way valve (13) is connected with the second two-way stop valve (18) by a pipeline, a port (i) is connected with the first two-way stop valve (17) by a pipeline, a port (g) and a port (j) are separately connected with the pipeline; the other ends of the first two-way stop valve (17) and the second two-way stop valve (18) are separately and directly connected with the openings for discharging and feeding the gas at the lower end and the upper end of the packed bed catalytic reactor by pipelines; (II) a heat exchange device comprising a raw material pre-heating heat exchanger and a recovery heat exchanger (10); the raw material preheating heat exchanger is installed on a pipeline before feeding the raw gas into the reactor unit; the recovery heat exchanger is installed on the pipeline after the product gas is discharged from the reactor unit, and is connected to the raw material preheating heat exchanger by a pipeline for pre-heating the raw gas by the waste heat carried by the product gas. The inert gas is silica, silicon carbide or aluminum oxide. During the use of flow direction change period operation reaction device, the three reactor units are connected by three combined manners where each reactor is similar to reactor unit (19). An INDEPENDENT CLAIM is included for methanating synthesis gas by flow direction change period operation reaction device: after being pre-heated to 80-250 degrees C by the raw material preheating heat exchanger, the raw gas is fed into the reactor unit; the ports (g)-(h) of the two-position four-way valve are communicated, and the ports (i)-(j) are communicated, at the same time, the communication relations between the ports (g)-(i) and the ports (h)-(j) are naturally disconnected, the raw gas flows from up to down in the packed bed catalytic reactor (5); when reaching the half-period time (t1/2), under the control of a computer, the ports (g)-(i) of the two-position four way valve are communicated, the ports (h)-(j) are communicated, at the same time, the communication relations between the ports (g)-(h) and the ports (i)-(j) are naturally disconnected, at this time, the raw gas flows from down to up in the packed bed catalytic reactor; by setting the half-period time (t1/2), the communication of the four ports of the two-position four-way valve is alternately and automatically controlled at regular time for periodically changing the flow direction of the gas in the packed bed catalytic reactor; the product gas from the reactor unit is treated by the recovery heat exchanger for recovering the heat and producing a by-product superheated steam, then the raw gas is pre-heated by the raw material preheating heat exchanger; the value of the half-period time (t1/2) is 5 minutes to 5 hour; the reaction temperature of the packed bed catalytic reactor is 250-700 degrees C, the pressure is 0.1-6.0 MPa, the space velocity of the raw gas is 2000-50000/hour. The methanation catalyst is an industrial nickel catalyst with nickel content of 5-20 wt.%.    DESCRIPTION OF DRAWING(S) - The figure shows flow direction change period operation reaction device for methanation of synthesis gas.    Port (g,h,i,j)    Packed bed catalytic reactor (5)    Heat exchange area (6,9)    Recovery heat exchanger (10)    Two-position four-way valve (13)    First two-way stop valve (17)    Second two-way stop valve (18)    Reactor unit (19)