The main function of the laser communication large-caliber ground optical transceiver is to establish a communication link with the satellite to realize data transmission between the satellite and the ground station. The 600 mm microcrystalline primary mirror of one laser communication station is heavy, and its working angle changes constantly. In order to decrease the mirror surface error, the support system not only has a 9-pose axial support structure but also simultaneously balances the radial component of gravity of the primary mirror at its working angle by using a radial support structure. Flexible lateral support structures have large size and stress, so it is not suit for the mirror that works in a wide range of rotation. The paper researches the lateral support structure with a mercury band and central shaft, and analyses the impact of mercury band parameters on the surface error. The designed lateral structure has small size and improves the surface quality of the mirror. The measured values of PV and RMS are smaller than λ/5 and λ/37, respectively. These result shows that the designed lateral support structure reaches the design purpose and satisfies the requirements.