To adapting to the execrable working conditions of a dynamic platform, a large-diameter Fast Steering Mirror(FSM) with a rigid support structure was designed. Firstly, the lightweight reflector, actuators, angle measurement elements,and the support structure of FSM system were designed and selected on the basis of the applied requirement of a vehicle theodolite for FSMs respectively. Then ,the reflector was lightweighted and the mode of support base was analyzed by finite element method. The moving parts of the FSM were connected with static parts by a spherical hinge and its main loads were carried out by the support base through the hinge indirectly. The design allows the FSM to bear large loads and to be suitable for the poor working conditions. Finally, the control system of FSM was constituted, and its control bandwidth and pointing precision were tested. The test result shows that the control bandwidth is 67 Hz, moreover , its pointing precisions for azimuth and pitching are 1.0″, and 1.1″, respectively. These data prove that the control system is stable and practical and can satisfy application requirements of vehicle platforms.