In order to solve the problem of low accuracy and robustness of power line extraction in complex environment, an automatic power line extraction method based on laser point cloud was proposed. The main direction of transmission line was determined by principal component analysis, and the long distance transmission corridor was divided into several spatial grids to deal with the disturbance of vegetation point clouds on the extraction algorithm when the terrain changes. Then a new top-down filtering algorithm was used to eliminate the object points in each spatial grid, and the automatic separation of power lines and towers was realized according to the difference of point cloud density distribution. In addition, a radius search algorithm is proposed to process the separated results and obtain the laser point cloud data of a single power line. The results show that the proposed method has a high accuracy of 99.69%. It has good robustness for different connection tower types and different terrain. This research has good engineering application value in the field of automatic analysis of transmission channel spatial structure and intelligent patrol inspection.