The invention discloses an existing railway center line extraction method based on point cloud data, and the method comprises the following steps: data preprocessing: carrying out the observation station splicing, gross error elimination and data cutting of original point cloud data; point cloud data filtering: filtering invalid point cloud data, and performing segmented filtering on the point cloud data according to the elevation of the rail surface; point cloud segmentation: performing gridding and binarization processing on the point cloud data, performing point cloud segmentation on the filtered point cloud data, and converting the processing of the point cloud data into the processing of the grid data; and line midline point calculation: calculating coordinates of the line midline point by utilizing the point cloud data of the two steel rails according to the parallel geometrical characteristics of the two steel rails of the existing railway to obtain a line midline. According tothe method, the existing railway line center line can be automatically extracted, the existing railway line center line measurement precision and operation efficiency are improved, and huge potentialsafety hazards caused by online operation of measurement personnel are avoided.