Traditional estimate method has limited capacity when the scan rate of radar antenna varies with time. Thus a state space model for estimating beam azimuth was established, and the particle filter was introduced to estimate the state variables. Particle filter is a powerful method for state estimation in non-linear and non-Gauss case， the radar beam′s azimuth estimating algorithm based on particle filter has better performance, and it can adapt to the case where the scan rate of radar antenna varies with time. The simulation results show that the applicability and the efficiency of the proposed algorithm are desired. 