The invention discloses a device and a method for converting methane in supercritical water. The device comprises a reaction device, a pipeline control device, a product collecting device, a temperature control device and a computer control device. The device is simple in structure and variable in function, methane and water in the same reaction kettle can react in the same environment, and different environments of methane and water can be controlled through different reaction kettles to react. Meanwhile, the temperature and the pressure in the reaction kettle are subjected to accurate information acquisition through the temperature control device, and the gas generated by the reaction is measured and analyzed after the test, so that the operation is easy, and the specific conversion process of methane in supercritical water can be comprehensively reflected.