Aiming at the problem of checking the overall length of step punch with circular section, the force of the step punch with circular section during blanking was abstracted as the model of the step bar under the action of axial force.A transcendental equation about the relationship between critical load parameters and section diameter ratio was obtained by analyzing the model, but the analytical solution could not be obtained.The numerical solution was obtained by using the univariate solution function of Excel software, and the relationship diagram between the diameter ratio of the step punch with circular section and the critical load parameters was drawn. The results show that when the diameter ratio b of step punch with circular section is in 0～1, the shape difference of the relationship curve between the diameter ratio of the step punch with circular section and the critical load parameters increases with the increase of the step punch length ratio a, and the curve shape takes on “leaf form”； at this time, the small end of the punch diameter is fixed, and the big end of the diameter is at the blanking end, so the punch is easy to be pulled out by the blanking material and causes accidents.When the diameter ratio b of the step punch with circular section is in 1～10, the shape difference decreases with the increase of the punch length ratio a；at this time, the big end of the punch diameter is fixed and the small diameter end is at the blanking end, so the punch is not easy to be pulled out. The above results fully explain that the rationality of the design for the step punch with large end fixed and small end blanking, which provides references for the step punch design