A full-wave equivalent circuit characterization strategy for large-scale microstrip transmission lines is presented based on the partial element equivalent circuit (PEEC) method. Specifically, the multilayered Green's functions are derived from the discrete complex-image method (DCIM), which consist of the quasi-static, surface-wave and complex images components. Considering time delays caused by electromagnetic wave propagation, the proposed full-wave strategy combined with PEEC can extract the equivalent circuit parameters efficiently, which provides clear physical insights, facilitating the field-circuit co-simulation and optimization. Finally, some numerical results are given to show the efficiency and accuracy of the proposed strategy.