To remove the influence of disturbances on the tracking accuracy of the stable platform,a new composite Sliding Mode Control (SMC) algorithm is proposed.Firstly,robust H∞ mixed sensitivity control is used to design a Robust Disturbance Observer (RDOB) and effectively optimize its filter,so as to estimate and compensate for equivalent disturbances rapidly and accurately and improve the disturbance rejection ability of the system.Secondly,in order to improve the tracking accuracy of the system,the finite time theory is used to design a New Sliding Mode Controller (NSMC),and the convergence of the system in a finite time is proved by using the Lyapunov function.The simulation results have proved the effectiveness of the control strategy.