Abstract: The traditional method to design interdigital filter, which obtains the filter parameters from calculating self-capacitance, mutual capacitance and Look-up table, proved to be complicated and inaccurate.In this paper, a new method utilizing external quality factor Q and the inter-resonator coupling coefficient K was introduced.As a sequent, an interdigital filter was designed for the microwave frequency-selector.The simulation results were compared with the experimental results.Further, the modulated method for bandpass filter was presented in detail.