Abstract: The frequency of the magnetron in a domestic microwave oven is often not fixed and difficult to measure. In order to obtain more accurate simulation results, the effect of microwave electromagnetic frequency fluctuation on temperature distribution is investigated. This study assumes that the frequency of the domestic microwave oven fluctuates around 2.45 GHz and compares the simulated based on the finite element model and experimental results of different fluctuation ranges ( 2.44 -2.46 GHz, 2.43-2.47 GHz, 2.41-2.49 GHz) and single frequency (2.45 GHz). The subjects are mashed potatoes heated in microwave oven for 60 s. The electromagnetic power density is calculated every 0. 005 GHz for different frequency range distributions and weighted averaged according to the cosine distribution, and finally the electromagnetic power is used as a heat source to heat the mashed potatoes. The simulated results are compared with the experimental values of the surface temperature and instantaneous temperature of the mashed potatoes measured by thermal imager and fiber optic thermocouple. The predicted temperature field in the frequency fluctuation range of 2.44-2.46 GHz is in good agreement with the experimental values. In the range of 2.41-2.49 GHz, the uniformity of temperature distribution is the best. The model can provide theoretical basis for the development of microwave food.