LAMOST (The Large Sky Area Multi-object Fiber Spectroscopic Telescope) is a reflecting Schmidt telescope. There are two large segmented mirrors in LAMOST: One is the Schmidt plate MA, and the other is the spherical primary mirror MB. The dimension of MB is about 6.7m×6m and it is face down in 25°. MB is composed of 37 hexagonal submirrors.During the observation, one should maintain the correct mirror figure for each sub-mirror and co-focus for all 37 sub-mirrors to obtain the good image, even it is an unconventional designed telescope without tracking movement on the primary mirror.

This paper presents the design and the finite element analysis for the whole primary mirror support system, which includes the optimization of the mirror support points distribution, the design and the testing of the prototype of MB sub-cell, the structure analysis and the design of the mirror support truss.