Based on the theoretical model of laser heterodyne detection system with Gaussian local oscillator beam and Gaussian signal beam,the impacts of spatial misalignment angle,deviation mismatch,optical path difference between the signal and local beams,the relative position of detector and the diameter of the photo-sensitive surface on the heterodyne efficiency and the amplitude of the heterodyne signal are investigated by numerical simulations.The results indicate that, the spatial misalignment angle and the deviation mismatch have great influence on the heterodyne efficiency and the amplitude of the heterodyne signal.The experiments about the influence of spatial misalignment angle and deviation mismatch on the amplitude of the heterodyne signal have been carried out.The experiment and simulation results give fairly good accordance.