Biogas digesters have recently been used as an alternative cooking fuel for some rural families in China. Biogas production by digesters is considerably affected by fermentation temperature. Soar energy can be used to improve the biogas production rates in winter in cold climate zone of China. A thermal balance model is developed to compute the fermentation temperature by ambient temperature and solar radiation for simple sunlight greenhouse assisted biogas project, a series of field experiments on a biogas project in Xuzhou, China was conducted to investigate the thermal performance of biogas digesters and to validate the model, the average temperature rise between the inside and ambient is over 10 °C in winter.