Heating uniformity is extremely important but difficult to achieve, and it is highly dependent on the characteristics of electric field. Hence, it is significant to study the condition of dramatic changes of electric field in multimode cavity. In this paper, firstly, Helmholtz equation and dyadic Green's function are used to derive the integral equation. Then, by discussing the characteristics of the derived equation, we obtain the main factors and characteristics of its stability. Finally, by finite element method, we verify the correctness of the conclusion, and present the smallest change of relative parameters that results in dramatic changes of electric field.
