To improve the analysis accuracy for the optical mechanical structure of a optical system, a nonlinear analysis method is introduced and then a nonlinear Finite Element(FE)analysis is applied to analysis of the mechanical model of a primary mirror subassembly. The main source for structural nonlinearity is summarized, and a FE model for the primary mirror subassembly of a collimator is established on the basis of contact theory. The mirror response based on bolt contact and the mirror deviation based on gravity and temperature loads are analyzed by contact nonlinear analysis method. Using a practical mesuring result, the linear and nonlinear methods are compared, and it is included that the contact nonlinear analysis indeed reduces the analysis error. Therefore, nonlinear analysis can improve the analysis accuracy and obtain the results which are close to that in real system of engineering analysis.