The nuclear reactor cooling medium drive motoris thecritical equipmentin the primary circuit of hightemperature gas cooled reactors. Ｒole of the helium gas as cooling medium drive system for the exchange of heat，In order to study the influence of characteristics of cooling medium and the fluid field distribution upon temperature rise and system stability in the motor. A 4 500 kW driving motor as the research object，considered the loss formula that helium as the coolant is different from the traditional formula，calculated the windage loss by using numerica method. Fluid field analysis model of the coolingsystem was established. Based on the principle of fluid force，the finite element method was used to obtain the distribution of fluid in the ventilation system; Finally，in order to study the characteristics of helium better，took air as a comparison object，analyzed on influence of fluid property difference upon the wind velocity distribution in the air gap，the rotor axial and radial direction of theventilation. The results of analysis shows that under the same conditions，the inlet-velocity of fluid region using helium cooling is lower 30% than that of using air cooling，which helps to reduce temperature rise of motor causedby windage loss.