Abstract: A giant steerable tertiary mirror system was promoted to ensure that the light gathered by the thirty-meter telescope (TMT) goes into the terminal equipment. This system contains two main assemblies:the mirror cell assembly and the positioner assembly. In the mirror cell assembly, both the axial support system and the lateral support system used the whiffle-tree structure. In the positioner assembly, an azimuth-altitude structure was used to solve the pointing problem. A detailed systemic parametric model was constructed in ANSYS. With the help of MATLAB and ANSYS, analysis under 98 main work conditions was carried out to verify the M3S performance. Some statistical methods were applied to process the simulation results. Then, the surface figure error was induced into the optical analysis. Result shows that the maximum displacement is up to 1.28 mm with a first modal frequency of 15.1 Hz. In optical analysis, the surface figure error degrades the modulation transfer function (MTF), however, the effect can be declined by active optics correction. Analysis results show that the proposed design can meet the requirements of the thirty-meter telescope project. Exploration of giant telescopes can significantly improve our country capabilities in telescope design and manufacture and accumulate experience for next generation of giant telescope construction.