The winding hot-spot temperature is one of the most critical parameters that determines the power transformer overloading capability and the aging rate of the oil-paper insulation. Based on the traditional thermal theory, the paper proposes a simulation model to predict transformer winding hot spot temperature taking into account oil viscosity changes with temperature, and combined with the thermal-electric analogy method and hot spot temperature rise model recommended by IEEE. The Runge-Kutta method is used to calculate top-oil temperature and winding hot-spot temperature of the transformer, and then compare them with the measured data obtained from the 100 kVA/5 kV (ONAN) experimental transformer. The simulation results are in good agreement with experimental data.