Abstract: Abstract: Anaerobic fermentation reactor is the core device of biogas projects. The study of low-energy anaerobic fermentation reactor in cold region is not only helpful to reduce the energy loss of biogas projects and to improve the output ratio of energy consumption and productivity, but also particularly important for the promotion and application of large and medium-sized biogas projects in cold northern China. The key issue about the low-energy anaerobic fermentation reactor is how to reasonably and effectively utilize "quantity" and "quality" of energy under the premise of ensuring the function of the reactor. In this paper the full hybrid anaerobic fermentation reactor built on the ground is taken as the research object. According to the method of energy analysis and exergy analysis, the author analyzes the energy-saving characteristics of low-energy anaerobic fermentation reactor in cold region in terms of the geometry analysis and the performance of envelope thermal insulation and the heating method of low-exergy. In this paper, the key problem of anaerobic fermentation reactor design is solved by using the concept of building shape coefficient. First, the shape of the reactor cross section is determined and 3 types of the reactor body shape coefficient models are established and solved by numerical analysis. The calculation results show that the optimal shape of the low-energy anaerobic fermentation reactor is cylindrical, and the aspect ratio is 1:2. Second, based on heat transfer theory, the thermal conductivity models of the difference of the volume of the reactor envelope structure are established by statistical method and numerical analysis. The results show that the limits of thermal resistance of heat conduction of the top and side wall envelopes of the energy-saving reactors in Harbin, Shenyang and Beijing under the outdoor temperature in winter were determined. When using other types of reactor materials, such as reinforced concrete, lipo tank, enamel assembly tank, and the insulation materials such as: foam perlite, polyurethane, etc, the thickness of insulation material can be converted according to the heat conduction and thermal resistance of the enclosure structure. Third, according to the method of exergy analysis, the heating system of medium temperature anaerobic fermentation reactor is analyzed and level analysis is used. Exergic losses caused by irreversible process should be reduced as much as possible. The calculation results determine that the inlet and outlet temperatures of the thermal medium of heating system are 45 and 38 ℃respectively, and the heating system efficiency is 86%, and the logarithmic mean temperature difference is 5.8 ℃. The model calculation results showed that the energy level difference between the energy supply and the energy consumption of the reactor is 0.018, which indicates that the energy levels can be matched and energy can be used efficiently. The conclusion can be used as a reference for energy saving design of low energy consumption anaerobic fermentation reactor in cold region.