In order to make the angles of aircraft aileron deflection can be controlled more accurately, an improved spatial four-bar (RSSR) mechanism is applied to the end of the transmission mechanism of the light aircraft aileron control system. The mathematical model of the mechanism is established by the method of direction cosine matrix, and the displacement formula of the mechanism is derived. In order to make an optimization design of RSSR mechanism, the parametric model of the mechanism is established in the software of ADAMS by estimating the initial value of the mechanism parameters, the objective function, the constraint function and the deriving function are determined, the sensitivity of each design point to the objective function was studied, the optimal design of the design variables with high sensitivity is carried out by the generalized reduced gradient algorithm. The results show that the aileron deflection angle has great influence by the relative position of each point of the RSSR mechanism, the relative error of the deflection angle of the aileron upward and downward are reduced from the initial 2.35% and 5% to 0.1005% and 0.1033% respectively, which demonstrated the optimization design for the mechanism meet the requirements of precision exactly. For the purpose of verifying the feasibility of the mechanism, the three-dimensional modeling for the mechanism is established in the software of CATIA, and used in a prototype of a 5-seat composite material light aircraft finally.