The invention relates to a methane two-phase anaerobic fermentation gas mixing system, which belongs to a methane generator. A gas returning pipe and a gas inlet pipe are connected and installed between a hydrolytic acidification tank and a gas production tank, the both ends of the gas returning pipe are respectively connected with the upper part of the hydrolytic acidification tank and the lowerpart of the gas production tank, the both ends of the gas inlet pipe are respectively connected with the lower part of the hydrolytic acidification tank and the upper part of the gas production tank;an air pump and a one-way valve are arranged on the gas inlet pipe; and a normally open solenoid valve is installed on a gas collector, and is positioned at the gas collector position between the hydrolytic acidification tank and the gas production tank. The system has the advantages that the structure is simple, the operation is convenient, the running is stable, the gas production process is smooth, the gas production rate is high, and the quality is good.