A numeric model is established for the microwave field during microwave heating by using Maxwell's equations. The coupling of equations of the electromagnetic fields and temperature fields is required. A two dimensional numerical model is developed to predict the distributions of electromagnetic field, power and temperature of high titanium slag positioned in the multi-mode microwave cavity. A three dimensional finite-difference time-domain (FDTD) method is used to show the influence of the substance by the electromagnetic field during microwave heating, by solving the transient Maxwell's equations and heat transfer equations. Transient finite difference method is used to obtain the temperature profiles. The comparisons of the theoretical results, using the proposed model, with experimental results, illustrate a very good agreement. The simulation results reveal that the model can be used to optimize the fundamental parameters and provide guidance for the treatment of high titanium slag by microwave heating.