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The wireless power transfer (WPT) technology has attracted more attention due to its convenience, electrocution free, and safety. However, the current discontinuity characteristics of WPT systems are not sufficiently appreciated. In this article, a unified full-load discrete-time (UFDT) model for the WPT system is established, which is both suitable for continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The state-space equation in each state interval is derived according to the equivalent circuits, and then the UFDT model in one period is obtained by iteration. Meanwhile, a closed-loop solution is investigated to calculate the duration of each state interval for different conditions. Based on the UFDT model, the characteristics of the series-series (SS) compensated WPT systems are analyzed and summarized among the full-load range, which provides a theoretical reference for solving the problems of WPT systems, especially in DCM with little theoretical basis so far. The state variables continuously change while the SS-compensated WPT system enters DCM from CCM with load resistance increasing. And the boundary of CCM and DCM is a multipeak function of operating frequency. Simulations and experiments are conducted to verify the UFDT model, and the full-load characteristics of the WPT system also match well with the experimental results.