For wireless power transfer (WPT) systems, the Series-Sires (SS) topology is widely used due to its simple structure and high efficiency. However, it is still a problem to realize zero-voltage switching (ZVS) and output rated power with misalignment and wide load range. To solve these problems, this paper analyzes input impedance angle θin, power and primary current characteristics with frequency modulation (FM). First, based on asymmetrical compensation factor τc theory, the demarcation line of θin has been identified, which reveals the distribution pattern of the power factor with different load resistance and frequency, aiding in the design of WPT system with high power factor. Then, the variation patterns of the primary current and system power have been found out, enabling the system to maintain a low primary current while delivering the rated output power. Finally, a 1.6 kW simulation model is designed to verify the theory, whose coupling coefficient varies in [0.15, 0.2] and load ranges in [15 Ω, 25 Ω]. Monotonic frequency control (MFC) can be achieved in constant current (CC) stage.