Abstract: Basic principle of the heterodyne efficiency of coherent Doppler wind lidar is briefly described in this paper. Under the condition of coaxial system, the heterodyne efficiency of local and transmit truncated Gaussian beams was studied. By using the sum of Gaussian functions to replace the circular aperture function, the analytical expressions of heterodyne efficiency were derived. Then, the dependence of heterodyne efficiency on the Fresnel number and truncation factor could be numerically analyzed using Mathematica software. The relation of maximum heterodyne efficiency with the change of Fresnel number was revealed and the values of optimum truncation factor under different Fresnel numbers were obtained. This research has important theoretical significance and practical value both for the research and development of coherent Doppler wind lidar.