The invention discloses a method for extracting opening amount of shield tunnel segments based on point cloud data, and belongs to the technical field of tunnel engineering survey. The method intercepts a tunnel section by a known plane, extracts cross-sectional point clouds of different rings according to the spatial relationship that a spatial cylinder intersects the plane, measures the arc length of the longitudinal seams at the splices of adjacent segments according to the abscissa of a scatter diagram, and determines the opening amount of the segments based on the difference between the arc length and the seam width designed in the segments. The method provided by the invention has the advantages of using the space cutting theory to extract the cross section of the shield tunnel basedon the three-dimensional laser point cloud data, being rather simple and high in efficiency and precision, making full use of massive point cloud raw data and other advantages, and being able to fastand accurately obtain the opening amount of lining segments at any length cross section of the tunnel through post-treatment.