This paper investigates the equivalent model for the friction boundary condition between blank and holder during sheet metal forming simulation, and proposes an equivalent algorithm, the thickness threshold value algorithm, for the first time based on thickness weighting.This algorithm assumes that the holder clearance is uniform during sheet metal forming; and the main reason for the unevenness of friction force distribution under the holder is that the uneven deformation of the blank leads to its uneven thickness distribution, which makes the local pressure on the blank distributed unevenly. The algorithm proposed in this paper can effectively simulate the influence of the unevenness on the forming process. Validity of this algorithm is verified by a comparison between the simulation results and the experimental ones for the drawing process of a car spring base.