To meet the command of detecting and inversion lidar dates, lidar crossover function must be calculated. In this article, a numerical method of calculating the lidar overlap function (geometrical factor) is presented.Based on our technical parameter of the lidar system, and starting from efficiently exploring backscattering echo which comes back from the 0～3 km aerosol,the influence of each system parameter on the overlap function and the way of changing the overlap function by altering the system parameters in particular are discussed.The feasibility study of optimal design of the lidar system is also carried out. The calculation and analysis results indicate that the method presented in this article is simple, intuitionistic and efficient.