The mathematic model of biogas heat pump with generator was established. It included biogas engine model, compressor model, heat exchanger model, generator model and etc. Energy supply performance for biogas heat pump with a generator was studied by using the models. The results show that the system has good power generation performance, heat supply performance and refrigeration performance. When the system runs at low speed, the efficiency of energy utilization is more, and the max COP of the system is about 5. The results prove that the system is with a high actively and fully utilizing energy technology. It has good partial load characteristic and good effects for energy saving and environment protection. The voltage of the generator is 28.8V, which is enough for the system operating.