In order to make better use of waste heat energy from engine exhaust gas, the waste heat recovery system based on organic Rankine cycle (ORC) was presented, a single screw expander was used as the power output device, R245fa was used as working fluid, the concept and calculating method of waste heat recovery utilization rate was introduced. On the basis of engine test results, the effects of working fluid evaporating pressure and engine speed on the ORC system performance were analyzed and discussed, and then the optimal value of working fluid evaporating pressure was selected. According to the research results, when the working fluid evaporating pressure is 3.0MPa, ORC waste heat recovery system can work properly over the whole range of engine speed, the maximum of net power is 12.1kW, the maximum of first law efficiency is 11.27%, the maximum of second law efficiency is 25.8%, the maximum of waste heat recovery utilization rate is 8.9%, waste heat energy from engine exhaust gas can be recovered effectively.