To improve dynamic and disturbance rejecting ability of radar stable servo system, output feedback H_∞ controller of speed loop was presented based on LMI. For signals with different amplitude and frequency, simulations of the closed loop in Matlab were done, which contained both step response and disturbance tests. Simulation results indicate that the servo system with LMI controller performs better in dynamic response with shorter response time. The new designed system has better rejection rate than the system with traditional PID controller for variable frequency disturbance signals. Also, the LMI controller advanced here not only has no shortcoming like saturation of controlled output but also has some advantages such as low order and easy to realize.