Abstract: The simulation calculation with the computer using FORTRAN language for frequency characteristics of the microwave duplexer tube (TR tube) is described in this pa-per. It is based on the theory of microwave four terminal networks with lumped parameter elements. The theoretical equations expressing the frequency charcteristics of TR. tube is given firstly. Then the effects of some factors, such as the staggered tuning, the length of coupling transmission line, and the element loss on the frequency characteristics are calcu-lated and discussed. The optimum values of staggered timing and optimum length of the line of a iour-resonate-elemeiit TR tube are given, and compared with the experimental data. In the program, the optimum parameters of the TR tube can be chosen automatically, and the curve of frequency characteristics is directly shown in the screen. The calculating model and method are also suitable for conventional microwave filter.