The invention provides a method for calculating a straightness of a tank body of a railway tanker and a method for calculating a difference between a maximum diameter and a minimum diameter of a section. The method for calculating the straightness comprises the following steps: receiving point cloud data of the tank body; performing fitting on point cloud of a cylindrical barrel body to obtain a fitting cylinder and an axis thereof; projecting a three-dimensional coordinate point satisfying a preset condition in the point cloud of the cylindrical barrel body to a plane, so as to obtain a projection point set, the axis is in a plane and the plane is at a target angle with a vertical plane; and calculating the straightness of a corresponding bus according to the projection point set. According to the method for calculating the straightness of the bus of the tank body, the geometric shape (contour) of the tank body is reproduced by fitting the three-dimensional laser-scanned point cloud,so that the straightness of the bus of the tank body is calculated, no manual measurement is relied on, and automatic measurement is realized.