In this paper, a thermal power system utilizing low-grade solar energy was proposed and analyzed. It can efficiently use the heat collected by the collector to provide electricity and also can provide some heat energy to satisfy the demand of heat. The system bases on the organic Rankine cycle. The evaporator employed in the system makes the organic working fluid turn to vapour phase at a high pressure level. Also a screw expander is employed in the system to conversion the heat energy to mechanical work. For the proposed system, a working condition was set and the performance of the system was calculated. When the temperature of the heat source is 95°C and the expander inlet pressure is 0.8MPa, the first law heat efficiency is 7.25% and the system exergetic efficiency is 10.14% for the system.