Abstract: Aiming at the extraction and reconstruction problem of power line morphological features in sag measurement, a method of power line sag measurement based on image coordinates mapping is purposed. The power line edge detection technology and the sag measurement principle are analyzed, and the mapping relationship between world coordinates and image coordinates is depicted. According to the internal and external orientation elements of the image, the projection transformation matrix of image points in the world coordinate system is calculated. The spatial relationship between power line and marking points in actual operation is obtained by simulating the established experimental model. In summary, the accurate measurement of the characteristic points of the power line sag is realized. The algorithm can effectively estimate the sag size by selecting the reference points of the power line tower mark reasonably. Experimental results show that the measurement error can meet the power line inspection in actual environment. The measurement methods presented can provide references for subsequent application research.