Abstract: The transmission of wave packets in a potential well was analyzed by the quantum effect theory. The occurrence condition of resonant transmission was obtained by classified discussion.The comparison of it with results of the numerical simulation and coupling experiment shows a general agreement. That is, the resonant enhancement will take place if the slot side length is an integral multiple of the microwave half-wavelength, when the direction of the incident electric field is perpendicular to the long side of the slot. In the meantime, it would help to illuminate strong couplings in high power microwave effect experiments on complicated electronic systems.