Abstract: Monopulse radar is one of the main measurement equipments in the exterior ballistic measurement system with high accuracy. In order to improve the accuracy, all measurement parameters are required to carry on wave refraction error correction. For the precision of refraction error correction on the velocity parameter measurement of monopulse radar is low nowadays, the paper points out a high precision refraction error correction method based on the velocity of direction cosine, which is on the basis of refraction error correction of distance and angle. First, according to the precise location of target after the refraction correction, the real velocity vector is obtained by utilizing the second order center smooth derivative method. Then we obtain the direction cosine from the object to radar station and geocentric by utilizing the locations of the survey station and geocentric. And then the target tangent direction cosine of the wave ray between the target and radar station is worked out. Eventually, we carry on refraction error correction of the range rate. The results show that the precision is up to 20% compared with the common direct derivative method, and the method possesses preferable practicability and effectiveness.