When utilizing laser Doppler effect to measure in plane displacement of an object at long distance, the intensity of Doppler signals of scattering light, SNR and quality of signals are very important parameters, and they affect the distance and accuracy of displacement measurement directly. The Doppler differential method, lock in amplifier and homomorphic filter are studied. A differential optical measuring system with the best effect, a lock in amplifier with high sensibility and high Q and a homomorphic system that can re construct the lost signals are designed. So the transverse displacement measurement of object at distance of 50m is accomplished with the maximal relative error of 1.5%.