In order to enable the car to be recognized by other vehicles when driving at night and avoid traffic accidents, a new type of rear position light optical system with a pattern on the condenser is designed. Based on the theory of non-imaging optics, the method of illuminance optimization is adopted and the coordinate value of each point of the free-form surface mirror is calculated by the formula. According to the light distribution requirements of the rear position lamp according to the laws and regulations, the reflecting surface is reasonably divided to meet the light distribution requirements. The light is traced by the Monte Carlo method. The experimental simulation results show that the light distribution effect meets the illuminance requirements of the national regulations GB5920-2008 for each test point of the rear position lamp. The new structure designed makes the light type more uniform and avoids bright occurrence of spots.