A novel passive location method is presented by using IR seeker image. As the seeker Field of View (FOV) is small and target distance is much bigger than target size, a monocular algorithm using characteristic lines to estimate target distance is developed based on the fact that the camera model can be described as weak perspective projection. Compared with traditional passive location method, the proposed algorithm utilizes the size and angle of characteristic lines in image to represent the variation of object attitude and the method improves the location precision and makes the location process independent on attitude estimation. Experimental results manifest that the proposed algorithm can locate the target accurately in low SNR, and the algorithm can adapt to the change of attitude.