An improved composite control algorithm to improve the pointing accuracy of a Acquiring,Tracking and Pointing(ATP) system in a satellite platform was presented for controlling a fast-steering-mirror to reduce the beam jitter. The advantages of the composite control combined an adaptive feedforward algorithm and a PID feedback algorithm were analyzed according to the two algorithms’ characteristics. Then,the convergence problem of direct composite control of two algorithms was analyzed and solved by introducing a new algorithm,decoupling composite control using decoupling theory. Finally, the algorithms referred above were experimented and compared. Experimental results indicate that,in the experiment conditions, the control accuracy of the decoupling composite control algorithm is nearly 10 times better than that of the classic PID control algorithm and 3 times better than that of the adaptive feedforward algorithm,respectively.It concludes that the decoupling composite control algorithm combines the merits of two algorithms and can reduce the beam jitter better.