In order to meet the increasing demand for beam quality of laser weapons, a design and aspheric optimization method of optical collimation system was proposed. This method was successfully applied to the lens design of a laser cauterization gun, so that the gun could obtain a higher quality collimating beam in a farther range. First, the initial structure was obtained based on the analysis of tactical and technical parameters and the calculation of the optical path, then the aspherical optimization was performed, the two simulation results were compared and analyzed based on ZEMAX. Numerical results show that the optimized structure has significant advantages. Its aspheric coefficient and curvature radius are 0.584 mm and 115.37 mm respectively, and the light energy utilization rate can reach 99.92% when the GB 1316-88 anti-reflection film is used. After tolerance analysis, the lens was processed and combined with the gun body for a board ablation experiment. The experimental results are in good agreement with the simulation results. The superiority of lens performance proves the feasibility and rationality of the proposed design and optimization method, and also provides a certain reference value for similar problems in other fields.