In order to analyse the motor fan cooling capacity under different structures and to optimize itscooling capacity，aY2-250M asynchronous motor was taken as an example. Not only was the motor external fan’sfluid fields calculated and analyzed，which was based on the theory of computational fluid dynamic( CFD) ，but alsothe cooling capacityof different structures fans were analyzed in detail as well. The results showed that theimprovement of fan structure can reduce fan wind friction loss， improve the cooling effect and operation efficiency ofthe motor in effect． We can complete the external fan optimization based on this conclution. This study providestheoretical basis for fan blades and its structural optimization，and the exploration of design method of motor’s newventilation structure is promoted effectively as well.