Considering the characteristics of the uniform motion of the missile and target before intersection,the target image measured with infrared seeker was used to analyze the passive ranging method based on target image area and target image gray-scale.The target image model was established.Two ranging equations based on image area gradient and image gray-scale gradient were deduced without target parameter.The correctness of the two ranging methods were verified by simulations.These two methods provide references for passive infrared image ranging.