With the ageing of the original railway and the increase in demand for capacity, the existing railway lines need to be altered, so the existing lines need to be surveyed without affecting normal operations. At present, most of the existing railway lines are surveyed by traditional manual measurement methods, which are inefficient, heavy, cumbersome and repeatedly on line. To address the above shortcomings, this paper uses a least squares based 2D template point cloud matching algorithm to jointly and repeatedly calculate the rail track surface centre point position and obtain the track centre line and track gauge and other related parameters based on the absolute 3D point cloud data obtained by the track mobile laser scanning system. The test and result analysis at the track test site show that the algorithm can effectively extract the track centre line and provide accurate centre line data for the existing line survey.