On the premise of ensuring reliability of insulation structure of stator winding, the insulation structure of ordinary winding is optimized by controlling the diameter of enameled wire conductor (negative tolerance) and the eccentricity (≤1.5) and thickness of paint film, and reducing the thickness of slot insulation and slot wedge, so as to improve the full ratio of stator slot and the motor efficiency. Thermal aging experiment is carried out for three insulation structures. The results show that the temperature indices of three insulation structures are all over 160 ℃, reaching 155(F) grade. The optimized insulation structure can help the design and development of high efficiency motor.