Landing is the most important stage of the Flight of UAV, the study of automatic landing of UAVs has important engineering significance. In this paper, the UAV landing trajectory is divided into approach phase, steep glide phase and flare phase; a cascade control structure controller of height tracking was applied and the landing control law was designed. The digital simulation was done in the MATLAB / simulink environment. The results of simulation indicated that UAV can track the designed landing trajectory very well under the control law of automatic landing and safe landing can be achieved.