To locate accurately a large aperture primary mirror with balance weight support and Whiffletree support in the primary mirror cell, a lateral positioning method for the large aperture primary mirror was proposed based on the kinematic constraint. The principle of kinematic constraint was introduced, a design scheme of lateral positioning system for the primary mirror was proposed including the selection of location points and the design of flexure hinges and location bases, and then a lateral positioning system was implemented. With finite element method, the influences of the lateral positioning system on the large aperture primary mirror system was analyzed in several aspects, such as system location, resonant frequency and mirror surface profile. As the results, it shows that the resonant frequency of the primary mirror reaches 13.6 Hz, the average displacement of the primary mirror is -355.863 nm along the Y-axis direction, and the primary mirror surface profile is uninfluenced. These parameters all meet the design requirements of this lateral positioning system. Experimental results verify the engineering signification of proposed lateral positioning system on the design of support and position systems of primary mirror in large aperture telescopes in the future.