A small vehicle-mounted, organic Rankine cycle (ORC) waste heat power generation system is built and optimized based on recovering expansion preheater after exhausting high temperature steam. Using the first and second law of thermodynamics, thermodynamic analysis, energy analysis and energy calculation of the small vehicle-mounted ORC waste heat power generation system were established. Taking R123 as working medium, heat source temperature 300℃, the thermal efficiency and total thermal efficiency of the system were calculated with given working medium flow and pressure. The total thermal efficiency of the system with/without preheater are 23.1% and 10.8% respectively. The heat transfer of the evaporator is 7.35kJ and 4.67kJ respectively. The results show that compared with the traditional ORC system without a preheater, the heat efficiency of the ORC system and the heat transfer coefficient of the evaporator are greatly improved.