In order to obtain the accurate surface error Root Mean Square (RMS) of the theodolite primary mirror, the research on the simulation method of sphere joints in the primary mirror supports was carried out.The detail finite element model of primary mirror supporting structures was established.A comparison was made among three simulation methods of sphere joints.They are the method of rigid connection, the method of restricting the freedom of nodes and the method of contact boundary condition.The surface error RMS of primary mirror tested by interferometer is 17.53 nm, while the simulated surface error RMS of the primary mirror is 16.37 nm.The deviation between simulated results and experimental results is 6.62%.The experimental results show that the method of contact boundary condition can obtain a more accurate surface error RMS of the primary mirror than the other two methods.The parameterized analysis was conducted on the structures of the primary supports by the method mentioned above.The influence of lateral supports′ location, lateral supports′ length and lateral supports′ width to the surface error of the primary mirror was obtained.It′s a good reference to the accurate analysis of the primary mirror surface error and the design of the primary mirror supports that simulating sphere joints using the method of contact boundary condition.