Analyzes the technical characters and working states of IR Imaging/MMW Radar combined seeker,introduces the method of calculating the angle between target and IR imaging sensor.Since the plane MMW Radar cannot supply the pitch angle of target,in this paper,using the altitude supply by altitude indicator in the missile,a new method is presented to calculate the pitch angle,and fuse the information got from IR and MMW system through the algorithm of optimal weighted mean,study shows that a better result can be get through this way.