The sag of conductor and ground wire is one of the key indicators for the construction quality and safety operation of the overhead transmission line. Aiming at the shortcomings of current sag measurement method’s accuracy and convenience, a mathematical model is proposed based on laser ranging and grating angle measurement technology for the sag measurement of the line. On the basis of this model, a data quality evaluation optimization algorithm which the similarity measure combined with the Newton interpolation method is put forward to realize the data set compensation and sag calculation for the case of insufficient robustness of the single conductor’s sag measurement of the multi-split line. Compared with point cloud extraction power line method, only a small amount of measurement data is needed for the proposed method to fit the line model and calculate the sag value. In order to verify the proposed method and compare with current sag measurement methods, a 220 kV double-split line is used in a test, and the result shows that the maximum sag error rate of the optimized data is only 1. 47%, which proves that the proposed method’ s measurement accuracy can meet the requirement of engineering sites, and can improve the security and efficiency of the overhead transmission line’s sag measurement.