With the aim to provide a high performance servo control for photoelectronic tracking systems, a simple Variable-structure PI controller(VSPI) is presented by combining the traditional PI controller theory and the variable-structure idea, then the principles of the controller are introduced and its design parameter selection is also discussed. Based on the structure of a photoelectronic tracking system , the servo control systems by using VSPI,Anti-windup PI controller(AMPI) and Integral Separation PI controller(ISPI) are simulated, respectively, and the experiments on positioning tracking and equivalent sine tracking are also carried out. The experimental result shows that the system with VSPI controller has no overshoot and its rise time, settling time and tracking precision are 0.378 s,0.488 s and 1.26″,respectively. Compared with the systems using other two algorithms, the system with VSIP can complete real time control and can offer better comprehensive performance, which completely satisfies the requirements of photoelectronic tracking systems for high precision and fast response.