In order to obtain the effect of supporting structures on the surface error of theodolite primary mirror, the topology optimization and parameter analysis of axial supporting structures of primary mirror were performed. First, contact boundary condition was used to establish the detail finite element model of primary mirror supporting structures. The surface error of original supporting structures was analyzed and the surface error RMS was obtained under both horizontal optical axis condition and vertical optical axis condition. Then, a 4D interferometer was used to measure the surface error RMS under lateral supporting condition. Results show that the deviation of surface error RMS of numerical results and experimental results is 13.2%, which verify the accuracy of simulation method. At last, the topology optimization of the primary axial supporting structures was carried out. The new axial supporting structures were made according to the topological configuration. After that, the parameter analysis was carried out on some important dimensions of axial supporting structures. Results show that the primary mirror surface error RMS of optimized supporting structures is obviously better than the original one. The original surface error RMS of axial supporting is 11.49 nm, while the optimized one is 8.38 nm. The research is an important reference to the design of primary mirror supporting structures.