The invention discloses a power transmission line sag measurement method and system based on an aerial image. The method comprises the steps of obtaining a plurality of power transmission line images;optimizing the three-dimensional space point coordinates and the camera parameters by adopting a light beam adjustment method to obtain optimized three-dimensional space point coordinates and optimized camera parameters; extracting power transmission line pixels in each power transmission line image by adopting a straight line extraction method to obtain a power transmission line binary image; constructing a cubic grid dot matrix in the three-dimensional space model of the power transmission line; wherein the envelope of the power transmission line is located in the cubic grid dot matrix; andaccording to the optimized three-dimensional space point coordinates and the optimized camera parameters, projecting the cubic grid dot matrix into the binary image of the power transmission line, and calculating to obtain the vertical arc value of the power transmission line. According to the invention, the cost is reduced, and the measurement precision is improved.