Abstract: The coaxial output-cavity with large coupling holes is often used in the high power microwave devices. The structure of this cavity is three dimensional with travelling wave boundary, and it is very difficult to calculate its high frequency characteristics by lumped parameters and numerical methods. On this condition, a kind of analytical method is developed in this paper. According to equivalent principle of field the cavity is divided into five regular regions. The field in each region is produced by proper surface magnetic current density on the common interface and can be expressed as integral of Green functions. The magnetic fields of two adjacent regions are then matched on the common interface to obtain a set of integral equations. From the integral equations the resonant frequency