In order to solve the problem of insufficient accuracy of image detection results obtained by traditional seam width change measurement algorithm under inclined conditions ,this paper proposes a seam width change meas- urement algorithm suitable for inclined shooting conditions. Triangular and square targets are used as left and right templates respectively. Firstly ,the appropriate size of the template is selected and the captured image is matched by template matching. Then ,the left and right templates are obtained and binarized to obtain two corner points of the left and right templates. The distance of the target is obtained by using the cross ratio invariance to obtain the width change of the seam. It is proved by experiments that this method is suitable for the measurement of joint width change under long-distance and inclined shooting. It has certain accuracy and reduces the cost of joint width change monitoring ,which provides certain value for engineering application.