In the paper, an open cavity is divided into two parts: closed part and open part. The closed part means the closed cavity, and the open part is equi valent to the absorbing traveling boundary. According to Maxwell theory, there exists a coupling equation between the modes of the closed cavity and those of the open part.Utilizing the method, the X-band six-cavity transit time oscill ator is calculated; the calculated frequency and the Q value of the working mode are very close to the measured values.