A computation method used for doubly reentrant cylindrical cavity of klystron is developed in this paper. According to equivalent principle of field the cavity is divided into three regular regions. The field in each region is supported by proper surface magnetic current density on the common surface and can be expressed as integrals of Green functions.The magnetic fields of two neighbouring regions are then matched on the common surface to yield a set of integral equations. By use of Galerkin’s method these integral equations are transformed into a set of linear algebraic equations from which the eigenvalue

i. e. resonant frequency of the cavity

and magnetic current densities can be solved. And then another important parameter R/Q may be determined on the basis of the field distribution. Some computation results of cavities with typical dimensions are given and compared reasonably well with experimental data. It is shown that this method is superior to other numerical method （finite difference method or finite element method） in the following aspects: simplicity

higher accuracy

less inner stores

and less computing time.