The Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) is a national large scientific project in China at the beginning of this century. It is an unconventional designed modern optical telescope and has the both large field of view and large aperture. The spherical primary mirror MB in LAMOST is a segmented mirror with 37 sub-mirrors. The MB will be supported by a very stable truss structure and the mirror surface will be kept in a high optical accuracy. This paper presents the work on the finite element model of the truss structure of MB and gives the results of static and dynamic analysis with this model especially for the optimization of the higher stiffness and the lighter weight.