The study focued on the influence of different step numbers on torque ripple and noise of motor. Through the Ansoft to calculate 3D electromagnetic field, and add the air gap flux density vector by 3D electromagnetic field calculation, using Ansys. The average of the air gap flux density after superposition to calculate by 2D sound field. To simplify the complex rotor step skewing 3D sound field calculation. Besided a simple process that the rotor could be divided into two segments skewing structure were put forward, the requirements met the process of design also summarized. Through experimental verification the simulation calculation and test were basically identical. To provide a reference for this kind of motor design and calculation.