TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred System: In the system, the inorganic salt separating system is installed at the bottom of the supercritical reactor (3), which is further connected with the conveying pipe (8) and the valve (V). The conveying pipe (8) between the high-pressure plunger pump (1), the supercritical reactor (3) and the condenser (4) is provided with a heat exchanger (2); the output port of the high-pressure plunger pump (1) is connected with a first heat-exchanging channel; water or the mixture of water and biomass output from the high-pressure plunger pump (1) is used as cold medium input into the heat exchanger (2); water or the mixture of water and biomass with exchanged heat is input into the supercritical reactor (3); an output port on the top of the supercritical reactor (3) is connected with the second heat exchanging channel of the heat exchanger; steam-air mixture output from the top of the supercritical reactor (3) is used as hot medium input into the heat exchanger (2) to be cooled and input into the condenser (4). The oxidizer pressure pump (6) and an oxidizer heater (7) are further installed in the production system; output port of the oxidizer pressure pump (6) is connected with the input port of the oxidizer heater (7); output port of the oxidizer heater (7) is connected with the supercritical reactor (3). Preferred Method: In producing methane, step (d) further comprises a step of opening two valves at the bottom of the reactor intermittently, and adding running water to separate inorganic salt from the bottom of the reactor. The step (d) further comprises a step of adding pressurized or heated oxidizer into the supercritical reactor. The step (e) further comprises a step of using pressurized water or the mixture of water and biomass to cool the steam-gas mixture output from the reactor, using the heat thereof to pre-heat pressurized water or the mixture of water and biomass, and condensing the cooled steam-gas mixture. The step (a) further comprises a step of filling some solid strong biomass into the supercritical reactor (3), and infusing water pressurized by the high-pressure plunger pump (1) into the supercritical reactor (3). The step (a) further comprises a step of filling the mixture of fragile biomass and water pressurized by the high-pressure plunger pump (1) into the supercritical reactor (3).