NOVELTY - The reaction device comprises a reaction unit, an upper layer, a middle layer and three reactor units. The reactor unit comprises one filling bed catalytic reactor, six two-way stop valves and a connection pipe. The filling bed catalytic reactor is divided into three layers. The upper layer is a heat exchange region for filing inert filler or solid catalyst with a reaction activity. The middle layer is a reaction region filled with the solid catalyst with reaction activity. The lower layer is a heat exchange region filled with the inert filler.    USE - The reaction device is useful for methanation of synthesis gas.    ADVANTAGE - The reaction device prolongs the shelf-life of the catalyst with high efficiency.    DETAILED DESCRIPTION - The reaction device comprises a reaction unit, an upper layer, a middle layer and three reactor units. The reactor unit comprises a filling bed catalytic reactor, six two-way stop valves and a connection pipe. The filling bed catalytic reactor is divided into three layers. The upper layer is a heat exchange region for filing inert filler or solid catalyst with a reaction activity. The middle layer is a reaction region filled with the solid catalyst with reaction activity. The lower layer is a heat exchange region filled with the inert filler or the solid catalyst with reaction activity. The upper and lower ends of the filling bed catalytic reactors are provided with an opening for gas. Two sides of the openings at the upper end of the filling bed catalytic reactors are provided with the first two-way stop valve (1) and a second two-way stop valve (2). The second two-way stop valve is connected by the pipe. Two sides of the openings at the lower end are provided with a third two-way stop valve (3) and a fourth two-way stop valve (4) connecting by the pipe. The first and the fourth two-way stop valves are located at the same side of the filling bed catalytic reactor. The second and the third two-way stop valves are located at the other side of the filling bed catalytic reactor. A fifth and the sixth two-way stop valves are connected with the openings for gas at upper and lower ends of the filling bed catalytic reactor through the pipes. The heat exchange device comprises a raw material pre-heating heat exchanger and a recovery heat exchanger. The raw material pre-heating heat exchanger is mounted on the pipe in front of the gas for entering the reactor units. The recovery heat exchanger is mounted on the pipe for the gas flowing out of the reactor unit and connected to the raw material pre-heating heat exchanger (9) for preheating the raw material after heating the products. The structure of the three reactor units is completely same as the reactor unit. The three reactor units are connected as one end of the first reactor unit. The first reactor unit is provided with a raw material air inlet while the other end inlet connected with the outlet of the second reactor unit. The outlet of the second reactor unit is connected with the inlet of the third reactor unit or directly connected with the pipe output product gas. The outlet of the third reactor unit is connected with the inlet of the second reactor unit through the pipe. An INDEPENDENT CLAIM is included for a method methanation of synthesis gas by the device.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of reaction device.    First two-way valve (1)    Second two-way valve (2)    Third two-way valve (3)    Fourth-two-way valve (4)    Preheating heat exchanger. (9)