Abstract: This paper proposes a method for the assessment of the sag of transmission line based on 3 D model comparison,which takes the laser point cloud data and BIM design model of the transmission line as the base for data fusion. First,we classify the original transmission line point cloud,establish a laser point cloud model of the transmission line,and convert the BIM design model into a BIM point cloud model. Second,we locate the laser point cloud model on the corresponding BIM point cloud model based on the inherent characteristics of the transmission line. An improved ICP algorithm is proposed to realize the precise matching between the laser point cloud model and the BIM point cloud model. Finally,the assessment results of the sag is achieved by model comparison. The experimental results show that this algorithm is operated without manual intervention and has the advantages of high efficiency and precision.