During the chemical reactions under microwave radiation，hot spots can usually be observed.Sometimes，hot spots may burn the reactants down or result in an explosion.A numerical model was presented to study the hot spots during the precipitation of calcium sulphate associated with microwave heating.Maxwell’s equations coupled with hydrodynamic equations and heat conduction equation were solved by using the finite-element method（FEM）.The calculated results were in agreement with the experiments.The results showed that the location of the hot spots was variable，which was related to the precipitant of calcium sulphate，and the hot spots often existed near the interfaces between the solution and air or the precipitant.