In this paper, a new two-step method is proposed to extract the characteristic properties of periodic slow-wave structures (SWS). The SOL calibration technique is used to eliminate the influence of error boxes at both ends of the circuit to obtain the transmission matrix of the core circuit. Then the characteristic properties including the complex propagation constant and the complex characteristic impedance are calculated. Following the calculation process of the proposed method, a co-planar slow-wave structure is studied in the paper as an example to verify the effectiveness of the method. Further, the input-output coupler is thereby designed based on the calculated results. The corresponding impedance matching is quite satisfactory.