In order to obtain rail longitudinal direction parameters of a rail profile measurement system, a rail longitudinal calibration method based on the planar target is proposed. The Z-axis of the target coordinate system is always vertical to the rail longitudinal direction when the plane target is close to the rail surface. The plane target is translated or rotated on the rail surface and is closely attached to the rail surface. A group of plane target images with different postures are collected by the camera, and the Z-axis unit vector of the target coordinate system is transformed into the world coordinate system. After the transformation, the coordinates of the unit vector are fitted to the plane, and the longitudinal direction vector of the rail is obtained by calculating the normal of the fitting plane, so as to complete the calibration of the longitudinal parameters of the rail. The effectiveness of the proposed method is verified in the laboratory environment, and the rail longitudinal calibration error is less than 0.1°. When the number of target images is more than 10, the optimal calibration effect can be achieved. This method provides a theoretical basis for the accuracy improvement and reliability evaluation of the rail profile measurement system.