A cascade sliding mode control method based on a variable gain reaching law was proposed to improve the target acquisition and tracking performance of a photoelectric tracking system. First, to reduce sliding mode chattering and improve the dynamic response speed, a novel sliding mode reaching law with variable gains was designed. The variable gains were designed based on the inverse sine and exponential functions. Then, based on the novel reaching law with variable gain, the speed and position sliding mode controllers of the system were presented. The speed and position sliding mode controllers include cascade sliding mode control, which was designed to enhance the dynamic and robust performance of the system and further improve the system target acquisition and tracking accuracy. Finally, a comparison was made between the control performance of a traditional cascade PI control and the proposed cascade sliding mode control, based on the azimuth axis of a photoelectric tracking system. The experimental results show that compared to traditional cascade PI control, cascade sliding mode control can reduce the acquisition time by 32% when detecting a target with a speed of 1 (°)/s. Moreover, it can reduce the RMS of the tracking error by 31% when tracking the position sine signal with a maximum speed of 4 (°)/s and a maximum acceleration of 2 (°)/s2. These results indicate that cascade sliding mode control can improve the control performance of the tracking system.