Sag of transmission line is one of the main indexes for circuit design, running and maintenance. It directly impacts on the safety of the operation. Analyzing the hanging state of overhead power line between the two towers, the overhanging state of line can be determined by an independent variable when the span length is certain. By analyzing the sectional power line image, the independent variable can be extracted and the aclinic catenary can be restored based on the catenary-segment. Then the sag would be calculated. Both two end contour line and non-contour line are simulated by the method. And the experiment had indicated that it is efficacious.