In order to meet the optoelectronic tracking requirements for a small size floating stabilized platform with small volume and high load inertia, an improved parallel mechanism based on deficient DOF parallel mechanism has been designed. The Jacobian matrix is derived, and the dynamic model has been established by using Lagrange method. Considering the internal coupling, uncertainties and disturbances of the system, a workspace based composite control with disturbance observer is proposed. The disturbance observer is used to observe the disturbance of the system and reduces the upper bound of the disturbance. Based on backstepping method, the sliding mode controller is designed for tracking and residual disturbance suppression. The simulation and experiment results show that the proposed model and method have made tracking error reduced to ±0.08° which is 14.5% of PID, and is suitable for parallel mechanism.