Coupling of scattering light in space into a single-mode fiber is a key technology in the process of developing the Laser Doppler Velocity Radar. In order to make sure that the radar can get the longest detect distance, we have discussed the method of determining the key parameters of the receiver/transmitter common-path optical system based on the singlemode fiber coupling, from the points of laser radar detect distance equation, principle of single-mode fiber coupling efficiency reach maximum, and considerations of laser transmitter. In the engineering development process of an laser Doppler velocity radar, we have designed the optical system which fulfills the requirement of detect distance bigger than 3Km, then given out simulation results of single-mode fiber coupling efficiency versus lens spherical aberration, fiber defocus and fiber tilt. The tolerance analysis result indicates that the coupling efficiency will bigger than 52% under the usual levels of optical manufacture and assembly. At last we designed specialized equipment for testing the single-mode fiber coupling efficiency of the system we developed, the results showed that after mechanical vibration and thermal experiments, the single-mode coupling efficiency is 55.9%, and in the operating temperature range of 20±3°C, the lowest coupling efficiency is 45%, which is still bigger than 35% as the system required to make sure the detect distance bigger than 3Km.