Abstract: In order to design a high performance laser Doppler velocimeter (LDV), the relationship between the signal intensity of laser Doppler and the diameter of spot was analyzed using the speckle theory, and the calculation formulas of the Doppler current was given based on functional theory. The polarization characteristic of scattered light and its intensity distribution were investigated. The results of theory and experiment show signal intensity of laser Doppler is proportional to the diameter of the active area in detector, and inverse proportional to the diameter of laser spot. By selecting circular polarization laser beam with smaller spot size and using detector of small active area to signal in specular direction, the signal to noise ratio and the detection capability of the system are improved significantly.