Abstract: In this paper, the FDTD method is employed to simulate the microwave pulse coupling into the nonmetal cavity with a slot. This paper analyses distribution of the coupling field and summarize the effect of relative dielectric con- stant, conductivity, dimensions of the slot and thickness of cavity. The results show that the coupling field at the axes keeps invariability, the coupling field on center-section is centrosymmetric field. The energies coupled into cavity become weak when thickness and relative dielectric constant increased, and it increases when area of slot increases or aspect ratio increa- ses. The coefficient of coupling electric field at center of slot linearly decreases when relative dielectric constant increases, the coefficient of coupling electric field at center of cavity oscillatorily decreases and reach maximum when relative dielectric constant is about 6. The coupling characteristic of magnetic field are alike to electric field.