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In this paper, the electromagnetic and temperature field of a 250-MW air-cooled hydro-generator in the Wuqiangxi hydropower station was calculated. The 2-D electromagnetic field model was built. Through solving the model, the additional loss and its distribution were determined. The rotor 3-D temperature field was then calculated and analyzed. Due to many gas eddies, the inner environment of a large hydro-generator is, therefore, complex. This leads to difficulty in determining the heat transfer coefficients on rotor surfaces. To accurately determine the heat transfer coefficients distributions, a method combining numerical and analytical calculations is proposed based on Fourier's theorem and Newton's heat transfer law. Using this new method, the heat transfer coefficients distributions on rotor surfaces were repeatedly calculated. The rotor temperature distribution was then computed. After comparing the measured values, the calculated temperature with determined heat transfer conditions were more accurate than that from traditional methods. This provides a theoretical basis for calculating heat transfer coefficients of outer surfaces of 1000-MW air-cooled giant hydroelectric generator.