Due to the structure and process control during the production process of tank containers, it is necessary to measure geometric dimensions, such as the roundness of the inner container. This paper proposes a method for obtaining the roundness of the inner container based on the three-dimensional laser scanning method. Firstly, the full-scale point cloud of the inner container is obtained by scanning; and the internal stiffener plates, pipes and other accessories point cloud are removed based on the normal vector algorithm. Secondly, the positive angle of the point cloud is calculated through the tube section ellipse fitting to achieve high-precision registration. Finally, using RANSAC central axis fitting and Euclidean distance calculation, the roundness of different cross-sections of the inner container point cloud is obtained. Compared with manual measurement, the accuracy of this method is better than 2 mm, and the measurement time is about 10 minutes, which improves measurement efficiency and reduces labor intensity. It provides a solution for 3D laser scanning for process measurement.