The remote sensing investigation ability of a mobile platform is promoted by setting a fast steering mirror which to reduce the servo error of inertially stabilized loop in the access of optical detection, then the platform′s pointing precision, track velocity and stabilized accuracy of line-of-sight will be improved effectively. A inertially stabilized gimbal and fast steering mirror were transformed into a parallel Dual-Input Single-Output(DISO) system. Based on the difference of actuator range and frequency between inertially stabilized gimbal and fast steering mirror, a combined control method was presented, which was used to assure the auxiliary pointing stability, and then the pointing precision and track velocity can be improved through this method. A simulation experiment was carried out to verify the effectiveness of the method which reduce the servo error of inertially stabilized platform from 0.018° to under 0.005° by putting in a fast steering mirror.