In order to establish the true shape mathematical model of the rail rolling contact fatigue (RCF) crack, the true shape point cloud data of the RCF crack at the rail gauge-rail shoulder was obtained by X-ray tomog- raphy. A method was proposed to model the shape of rail RCF crack by using the grid algorithm to determine the boundary point of crack tip and opening, fitting crack tip with cubic B-spline curve and using intersection of crack surface and rail surface to determine the shape of crack opening. It was found that the number of crack boundary feature points extracted by the grid algorithm was more than that extracted by the convex hull algo- rithm. Both the bumps and the concave points representing the crack boundary feature were identified by the grid algorithm and the crack boundary distortion was smaller. As the length of the grid was gradually reduced, the number of crack boundary points extracted by the grid algorithm was further increased and the missing point problem of the convex hull algorithm was solved. The extracted crack tip boundary points were fitted by the least square method, Bezier curve and cubic B-spline curve. It found out that the crack tip obtained by the cubic B- spline curve fitting was the closest to the real crack and the maximum deviation and standard deviation of data fitting with the cubic B-spline curve were both the smallest and the mathematical model of the true crack tip shape was obtained.