The RF field in the RF cavity can be expressed by the modes of the cavity. And the characters of the cavity is determined by such modes as the frequency and field. In the paper，an actual cavity is divided into two parts: the normal part (such as a normal cylindrical cavity) and the abnormal part (including a coupling hole). According to Maxwell theory，there exists a coupling equation between the modes of normal part and the modes of the cavity. So the influence of the coupling hole can be studied. Utilizing the method，an L-band cylindrical cavity with a coupling hole is studied，the calculated frequency of the working mode is very close to the simulated value.