In the premise of the measured circulation flow rate, the conjugate heat transfer in turbulent flow within one coupled rotor model of a 250-MW hydrogenerator was calculated. The flow rate changes of the fluid in the support, yoke ducts, pole gap, and the air gap between stator and rotor were researched using the finite volume method, also their effects on the operating rotor temperature. The analysis shows that it can accurately simulate the fluid field of the rotor ventilation system, which will provide theory basis for the design and improvement of the generator ventilation system. The calculated average temperature of the excitation winding is compared with its measured value, and the error is less than 1%. Thus, the results can be used to simulate temperatures of the rotor which cannot be easily measured.