It is well known that laser possesses high brightness, high coherence, good directivity and other unique properties. In many practical applications, it is necessary to get small light spot in intermediate and long distance. Intermediate and long distance laser measurement demands to minimize the spot radius in order to improve the spatial resolution of the system and signal quality. Therefore, the study of Gaussian beam focusing property has high value for practical applications. In order to achieve intermediate and long distance laser measurement, this paper studies the method to adjust Gaussian beam spot diameter within a certain range after a near-field optical system transformation to improve the signal quality. Based on the fundamental characteristics of the Gaussian beam, this paper deduces the theoretical formula for the position and radius of the Gaussian beam waist and measures them by means of the CCD method. Then Matlab is used to simulate the spot diameter in the far field, and by combining numerical simulation results as well as optimizing the structure of the actual optical system, we make the light spot diameter in the target area fit the requirements of the laser velocimeter in intermediate and long distance measurement.