Vacuum freeze drying technology is often used to produce high quality dried white mushrooms, but its disadvantages of high energy consumption and long drying time limit its application. Compared with conventional freeze drying technology, microwave freeze drying can greatly save drying time and energy consumption, while retaining the characteristics of the freeze dried products. The process of heat and mass transfer during microwave freeze drying is very complicated, and the precise prediction of the heat and mass transfer is important for control of the drying process. In addition, international and domestic reports on the simulation of heat and mass transfer during microwave freeze drying have not considered that a material’s dielectric loss factor can lead to variation in microwave absorption ability, resulting in imprecise simulation results. In this study, a vector network analyzer was used to determine the dielectric properties of the mushrooms and a regression equation of dielectric loss factor versus temperature and moisture content was generated. Based on the dielectric property, a numerical simulation of heat and mass transfer during microwave freeze drying was carried out using a commonly used sublimation-condensation model, and was experimentally verified by conducting microwave freeze drying on white mushrooms. The results show that the simulation of heat and mass transfer based on dielectric property can precisely predict the temperature distribution of the samples during microwave freeze drying.