To address the issue of large error when using horizontal 2D midline to extract the cross-sections of large longitudinal slope tunnel, an effective method for extracting the 3D axis and cross-sections of the tunnel is proposed. First, the 2D boundary points of the tunnel are extracted using boundary grid and angle constraints. Then, the boundary points are fitted using iterative least squares with multi-conditional constraints to extract horizontal and lateral midline points. Next, the lateral midline points are orthographically projected on the vertical curved surface containing the horizontal midline to obtain the 3D axis points. Finally, the infinitesimal method is used to calculate the normal plane equation corresponding to the 3D axis points and a point cloud with a certain thickness on both sides of the normal plane is projected onto this plane to extract tunnel cross-sections. The experimental results show that the proposed method can extract the 3D axis and cross-sections of the tunnel point cloud with high accuracy. Furthermore, the cross-sections extraction error is less than 3.06%; thus, the proposed method is applicable for straight and curved tunnels with varying longitudinal slopes.