In order to solve the problem that the antenna pattern of secondary radar is difficult to be measured when the antenna is not easy to rotate or move in an environment like the secondary radar position or the outfield test, a method of antenna radiation pattern test based on the mobile testing vehicle is adopted. This paper analyzes the principle of far-field test of conventional antenna, and elaborates the test principle, system composition, procedures and error calculation of the method in this paper, and the mounted antenna pattern is tested in the outfield. The results show that the method is basically consistent with the test results of the dark room, and can solve the problem of antenna pattern test in the confined external environment above.