An improved method to design equivalent load for multi-stage planetary was introduced in this paper on the basis of methodology for determining the equivalent torque. A multi-stage nominal-load representation of designing equivalent load for load spectrum on each stage was proposed. Compared with traditional single-stage nominal-load method of a gearbox for wind turbine pitch control, it was showed there was an effectiveness and reasonableness of the proposed method in this paper. A testing rig was set up for a pitch drive which it was designed by the multi-stage nominal-load method. A lot of experiments were carried out, and the excellent performances of the pitch drive were showed. From the result, it can be seen that this improved method was feasible.