The embodiment of the invention provides a sag measuring method and device, and belongs to the technical field of electric power transmission. The method includes the following steps: obtaining a horizontal distance between a first suspension point and a second suspension point; obtaining a first tangent inclination angle of the first suspension point and a second tangent inclination angle of the second suspension point; and on the basis of the horizontal distance, the first tangent inclination angle, the second tangent inclination angle and a first preset regulation, obtaining sag of a power transmission lead. In this way, the sag of the power transmission lead is measured accurately.