The coupling effects of HPM to slots with different size were tested and the general coupling characteristics were obtained. The results indicate that the coupling of narrower slots have stronger polarized of electric field. When the lengths of the slots are equivalent to the wave lengths, the coupling effect is the strongest. In the range of band UWB, L, S and X, the narrower the slot is, the weaker the coupling effect is. The coupling effect becomes weaker when the slot becomes deeper. The pulse width of radiation waves has little influence on the coupling effect.