NOVELTY - A three-stage constant temperature system comprises high temperature anaerobic fermentor (1) connected with medium temperature anaerobic fermentor (2) through second electromagnetic valve (1-2). The medium temperature anaerobic fermentor is connected with constant temperature anaerobic fermentor (3) through third electromagnetic valve (1-3). The high temperature anaerobic fermentor and medium temperature anaerobic fermentor are respectively connected with heat exchanger (4) through third valve and second valve.    USE - A three-stage constant temperature system for producing biogas.    ADVANTAGE - The system realizes graded use of hot water energy; and has large gas production rate and stable gas production.    DETAILED DESCRIPTION - Three-stage constant temperature system comprises high temperature anaerobic fermentor (1) connected with medium temperature anaerobic fermentor (2) through second electromagnetic valve (1-2). The medium temperature anaerobic fermentor is connected with constant temperature anaerobic fermentor (3) through third electromagnetic valve (1-3), and provided with second temperature sensor (3-2). The high temperature anaerobic fermentor and medium temperature anaerobic fermentor are respectively connected with heat exchanger (4) through third valve and second valve. The constant temperature anaerobic fermentor is connected with heat exchanger connecting high temperature anaerobic fermentor through first electromagnetic valve (1-1), and provided with third temperature sensor. The high temperature anaerobic fermentor is provided with first temperature sensor (3-1). The three fermentors are respectively interconnected with first valve (4-1).    DESCRIPTION OF DRAWING(S) - The drawing is a perspective view of three-stage constant temperature system.    High temperature anaerobic fermentor (1)    First electromagnetic valve (1-1)    Second electromagnetic valve (1-2)    Third electromagnetic valve (1-3)    Medium temperature anaerobic fermentor (2)    Constant temperature anaerobic fermentor (3)    First temperature sensor (3-1)    Second temperature sensor (3-2)    Heat exchanger (4)    First valve (4-1)