Abstract: Based on generalized theory of transmission line, a set of partial differential equations of first orderare obtained to study characteristic parameters of cavity and beam-wave interaction of gyrotron. A calculationcode including "cold cavity" and "hot cavity" is developed. Taking the advantages of the code and commercial software, characteristic parameters of the same line-joint cavity are simulated. Correctness of the code is proved by simulation results. Purity of operating mode and efficiency of beam-wave interaction in two kinds of cavities, namely line-joint structure and arc-joint structure, are studied by the code when TE_{22, 6}mode is regarded as the operating mode of a110 GHz gyrotron. As a result, compared with the line-joint cavity, purity of the operating mode and efficiency are raised about 4.7 dB and 4%, respectively. In the gyrotron with an arc-joint cavity, an output power of 1.9 MW, corresponding to 50% efficiency and an oscillation frequency of 110.1 GHz, has been achieved with a 96 kV, 40 A helical electron beam at a guiding magnetic field of 4.41 T.