Aiming at the problem of detuning and control accuracy in the tunable filter, a novel design method of the coupling structure with constant bandwidth is presented in this paper. The cosimulation process of using the 3D electromagnetic field simulation software HFSS is discussed, and the key software and hardware technologies of the motion actuator mechanism are analyzed. A Cband tunable filter with high power of 400 W is designed. The simulation and measured results show that the bandwidth of the filter is constant in the whole tunable band is nearly constant，and the specifications of the standing wave，suppression and insertion loss are stable and good.