The development process of YX400-4 6 kV level 1 energy efficiency high-voltage box motor is introduced. The energy efficiency grade of the motor is promoted to National level 1 by the electromagnetic optimization, the thickness reduction of insulation, and the structural improvement especially that of the ventilation structure. Development results verify the effectiveness of the above measures. Higher efficiency of motor, which has surpassed the energy efficiency grade of National level 2, is achieved. It has outstanding performance in energy saving, pollution reduction and environmental protection.