In this paper, an improved method is proposed to extract the coupling coefficient between the resonator and patch for compact filtering antenna design. The proposed technique is based on the intrinsic properties of a general second-order band-pass filter network, which is the equivalence of a resonator-fed patch antenna. Theoretical analysis demonstrates that the S-parameters of such a two-port network is just a function of the coupling coefficient between resonator and patch if the external quality factors are given. To verify the validity of the proposed method, two types of resonator-fed patch antennas are designed under different coupling strength in this paper. The first antenna is fed by a half-wavelength resonator beneath patch, and the coupling strength is controlled by altering the shape and effective coupling length of resonator. The second one is fed by a rectangular ring resonator covered by patch, and the coupling strength is flexibly manipulated by changing the width-to-length ratio of resonator. The coupling coefficients are figured out when the simulated results fit the calculated ones from circuit model. The proposed method makes it possible to extract coupling coefficient between loss and lossless resonators within complex boundaries, which will felicitate the design of advanced filtering antennas with much more geometrical freedom.