Radar and IR image sensor both have disadvantages because of their intrinsic characters. For improving the target tracking performance, an IMM target tracking algorithm is proposed to synthesize the information from both of them. In this algorithm, IR image was processed by digital image processing technology firstly. Then its result was fused with the radar information in the algorithm of IMM target tracking. At last, the final tracking result was got through distribution method. This algorithm can reduce operating time and improve the tracking precision.