In this paper, a thermal power system utilizing low-grade solar energy was proposed. It can efficiently use the heat collected by the collector to provide electricity and co-provide certain amount of domestic hot water. The system bases on the organic rankine cycle, and employs an evaporator to receive the heat from the heat collector. That makes the organic working fluid turn to vapour phase at a high pressure level. And then the heat loaded by the working fluid is transferred to power by an expander or a turbine ，which is placed in between the evaporator section and condenser section. After that, the fluid flows into the condenser and is condensed at a low pressure level. By a pump the fluid flows back to the evaporator and a new cycle begins. Based on the models, the heat balance calculation and exergy banlance calculation of the system have been performed. And the thermal efficiency and exergy efficiency of every part and the entire system is obtained.