In the accident of pumping unit，the premature failure of the crank pin amounts to more than 30%. The improvement to the crank pin, using conventional analysis method, cannot solve the premature damage problems. Fretting wear theory was used to analyze the characteristics of the crank pin damages, and that the premature damage of the crank pin was associated with the fretting could be gained. In order to prolong the service life of the crank pin, excepting the diameter, length, pitch determined by the mechanical system and structure requiring, more attention should be paid to the parameters of other small parts, such as the size and the transition structure of the clearance groove on the pyramidal face, conical taper, and the radius of thread tool withdrawal groove, etc, which deeply influenced it. At the same time, the special technical requirements of the crank pin should be assured during the course of working.