It has been proved that a subwavelength periodic microstrip structure can be used to suppress crosstalk between two adjacent microstrip lines and such a microstrip structure can be widely used in the application of high-speed circuit systems. To further increase the accuracy of the equivalent circuit model of the subwavelength periodic microstrip line (SPML), we focus our attention on a single unit cell with the periodic structure, which is divided into several sub-regions, and the circuit parameters in each sub-region are analysed for setting up a complete circuit model of the unit cells. Then the details of the interaction between each sub-region and the electromagnetic field are clearly indicated. The S $S$ -parameters of the circuit model accord well with full-wave simulation results, showing a maximum deviation of 0.03973 dB for S21 and 0.1211 dB for S11 below 15 GHz. For the first time, we also study the frequency-dependence of the mutual capacitance and the mutual inductance for analysing the coupling effect between an SPML and a conventional microstrip line (CML). The instantaneous impedance obtained by the full-wave simulation of the SPML is consistent well with the experimental results.