Abstract: Based on coherent Doppler LIDAR heterodyne detection theory, the heterodyne efficiency analytical expression under both free-space and fiber-coupled detect mode of the coaxial system were unified through the parameter substitution. Combined this formula with the signal-to-noise ratio under turbulent environment, the optimum values of the aperture and the truncation factor under different detection range were obtained for ground based and spaceborne system. The results show that the signal- to-noise ratio of the ground based LIDAR is relatively steep as the telescope aperture changes, the optimum truncation factor is 80% during the whole detect range, and if the aperture selected is not appropriate, it will cause losses of signal-to-noise huge in the system; For Spaceborne LIDAR the signal- to-noise ratio is first increased and then stable as the telescope aperture increased, the optimum telescope aperture can be balanced between the cost and signal-to-noise, and the optimum truncation factor is also 80%. The research has important theoretical significance and practical value both for the development of detection theory and optimal configuration of the coherent Doppler LIDAR.