The temperature uniformity of materials in the process of microwave heating is a hot issue. A numerical calculation of the motion state of the microwave heating material model is proposed for the mechanism for “thermal runaway” phenomenon, based on the facts that the relationship between the resonant cavity of microwave electromagnetic field amplitude and phase are derived from the microwave transmission line and the generalized transmission line theory, and the relationship between the distribution of transmission line voltage and current. The change of dynamic mesh boundary tracking solution is used in this model, the control function for internal domain and boundary grid movement and deformation is proposed, which not only can solve the problem of the grid cross winding solving domain in mobile grid resulted from numerical calculation of heating process, but also can reduce effectively calculation of the mesh mobile node under meeting the calculation accuracy. The numerical results show that the temperature uniform of the motion state of the material is better than that of the static state of the material using microwave heating, and that verify the feasibility and effectiveness of the proposed numerical calculation model. The experimental results show that the proposed method can inhibit the mutations of the microwave heating temperature.