Abstract: Cauchy method can only diagnose the bandpass filters without source-load coupling. To solve this problem, a method is proposed for diagnosing cross-coupled resonator bandstop and bandpass filters with source-load coupling. Firstly, an optimization method based on the genetic algorithm (GA) is proposed to determine source-load coupling coefficient, and to remove the phase-shift effects on the simulated S-parameters, which are caused by the transmission lines at the input/output ports of a filter. Secondly, the characteristic polynomials of the S-parameters can be solved in one step using the Cauchy method. Finally, the N+2 coupling matrix of a filter can be extracted from the characteristic polynomials and source-load coupling coefficient using well established techniques. The proposed method can reveal what is wrong with the filter and its tuning direction by comparing the difference between the ideal and the extracted coupling matrix. Therefore, the proposed method can guide the process of the filter tuning which can speed up the design and realization of a cross-coupled resonator bandstop and bandpass filter.