The work aims to study the changes of temperature and moisture loss of porous media food with different water contents by microwave heating. A three-dimensional porous media finite element model of coupled electromagnetic field, flow field, heat and mass transfer was established in COMSOL Multiphysics 6.0. The accuracy of the model was verified through experiments. The temperature distribution of mashed potatoes with different volume fractions, the transient temperature change of characteristic points and the change of moisture loss were predicted. The results showed that the temperature distribution on the surface of mashed potatoes in 600 s was consistent with the experimental results, and the RMSE of the cold point and hot point were 1.23 ℃ and 1.92 ℃, respectively, with an average error of 3.9% and 3.7%. The RMSE maximum value of the simulation and experimental results of the transient temperature changes of the five feature points was 1.146 ℃, and the RMSE value of the temperature uniformity coefficient COVT was 0.008 1. The RMSE value of simulated moisture loss changes and experimental results was 0.963 1 g, and the average error was 3.69%, which indicated that the simulation model was reliable. Based on this model, it was found that with the increase of mashed potato volume fraction, the temperature of the cold hot spot presented a nonlinear increase. The transient temperature of 5 characteristic points at different volume fractions increased nonlinearly before 360 s, and became stable after 360 s. The moisture loss increased significantly after 60 s and linearly after 360 s. Volume fraction and heating time have obvious influence on the microwave heating process of mashed potatoes. It provides a theoretical basis for food packaging developers to understand the microwave heating process of food from space and time, and provides help for food packaging materials or structural design.