Regular monitoring of the size of wire sag in transmission lines is of great significance for the stable operation of transmission lines. The traditional sag measurement method is complex and time-consuming， and obtaining a complete wire image using monocular vision is a convenient sag measurement method. However， when the span of the transmission line is too large， the camera is limited by the field of view angle and resolution， making it difficult to capture the complete wire and measure the sag. To address the above issues， a convenient and usable local photogrammetry method was designed， which combines the accelerometer sensor in the IMU with a high-resolution industrial monocular camera. With a small amount of transmission line parameters known， only the left part of the tested wire needs to be photographed. After obtaining a single wire image， semantic segmentation technology was introduced to extract the wires in the image. Based on the photogrammetric model， the actual points of the wires were connected to the corresponding imaging points， and a nonlinear equation system was established. After nonlinear solving， the three-dimensional shape of the wires was restored， and then the wire sag was calculated. The relative error of sag measurement can be controlled within 5%， meeting practical application requirements.