A new method which depended on the resonant frequency and unloaded Q to accurate compute the physical parameters of the stepped impedance resonators(SIR) coaxial cavity is proposed and the detailed design process is given in this paper. This method was not only beneficial for reducing the design time of the filters, but also simplified the design process of the filters. The method is simulated and verified by the software HFSS. Compared with the design method which put forward by M.Makimoto etc, the precision doubled and the simulation results show that the error of the resonant frequency is less than 1%, the error of the unloaded Qis less than 8%.