Abstract: Two kinds of solar-sail attitude control mechanism were described in this paper, one was the centroid offet class, the other was the sail plane rotating class. Advantages and disadvantages of these two kinds of attitude control mechanism were analyzed. An attitude control roll stabilizer barsis was designed which was suitable for a kind of square solar sail which had extension boom and square sail surface. The boom's length of roll stabilizer bars is the key parameter, it was optimized under some constraints such as: configuration design, material strength, structural strength, quality optimization and control accuracy, it was calculated and verified through geometry relations and nonlinear static finite element simulation. The simulation result was 1.2 m. The rotate speed limit was obtained by power and speed relation formula according to the result of the boom's length, which was 20.83 ()/s. In this paper, an optimization design idea of roll stabilizer bars boom's length was given. The calculation and simulation results provide the key design parameters and design conditions for the solar-sail spacecraft attitude control system design, and the optimization results have reference value for practical engineering applications.