In this study, we propose an improved cavern capacity calculation model to address the defects of existing point cloud methods. First, the cavern preprocessed point cloud is sectioned into several point cloud slices from the bottom to the top along the height direction. For each slice, the contour points of two cross sections are extracted using the presented hierarchical projection method,and the section contour polygon is constructed correctly by using the improved ordering algorithm for section contour points. Subsequently, the area of each cross section is calculated using the coordinate analytic method. Given the structures of the caverns, the cell volumes of the lower side wall and upper arch of caverns are calculated by using models of cylinder and trustum of a pyramid, respectively. Finally, the capacity table of the caverns can be acquired based on the volume of every polyhedron from the bottom to the top. The performance of the proposed method is evaluated using three representative point cloud fragments from a cavern. The results denote that the accuracy of the output capacity table obtained by experiments is an order of magnitude higher than that demanded by the standard requirement. The proposed approach shows good adaptability with respect to the complex structure of caverns.