According to 1.2 m large aperture telescope primary mirror support system, an effective assembly method was advanced to insure the root-mean-square (RMS) of the surface accuracy. The primary mirror support system was designed following kinematic principle. To achieve excellent performance of the RMS of primary mirror surface accuracy at different ambient temperature (from -20 ℃ to 60 ℃) and different elevation(from vertical to horizontal), whiffletree structure were used for axial support,while lateral support mechanisms based on flexible tangent link structure were adopted. Resulting from machining error and installation error of the flexible structures, assembly stress was presented certainly, that could increase primary mirror distortion range evidently. By the finite element analysis(FEA) simulation, the effects of various errors were calculated. According to simulation results, assembly processes were established. After assembly of primary mirror support system, the RMS of primary mirror surface accuracy was measured by compensator and laser interferometer. In measurement, the RMS of surface accuracy was λ/42 and λ/31(λ=632.8 nm), when optical axis of primary mirror was vertical and horizontal.