This paper mainly focussed on the Simulation design of the coaxial resonator diplexer using the application software. Firstly, the simulating result was obtained by Designer software and suitable quantity of the resonantor was attained. Secondly, the equivalent circuit model was built using the Microwave Office software. The coulpling coefficient was obtained by optimizing the simulation to fit it into the technical specification. Thirdly, the single cavity model is built using the HFSS software and simulated untill the eigen mode closed to the center frequency of passband and the larger value of unloaded Q. the double cavity model was build based on previous simulation results. The dimension of the coupling window was adjusted during the simulation process until the required coupling coefficient was met.The diplexer was fabricated according to above design parameters.