Abstract: In view of demand of temperature and taxi gas in Ningxia，constant temperature system of biogas fermentation tank was designed by using model of gas producing-heat-constant temperature- gas producing process.The straw was fermented to produce methane，the methane was pressurized into natural gas to be used for taxi gas，the partial methane was burned to generate heat for preheating fermentation liquid，heat loss of circulating water pipe was reduced，and heat dissipation loss of fermentation tank was reduced.And it was compared with hot water cycle heating method commonly used in large and medium scale biogas projects both at home and abroad.Results showed that biogas fermentation tank constant temperature system（method of preheating fermentation liquid outside tank）could reduce use of biogas and other energy sources，and amount of methane needed was 15.3% of total gas production.