A resonant converter and its controlling method are provided. The resonant converter includes a bridge switching circuit receiving a DC input voltage through its power terminal, a resonant and transforming circuit, a rectifying and filtering circuit, and an over-current protecting circuit. The resonant and transforming circuit has at least one resonant capacitor charged/discharged in response to the switching of the bridge switching circuit. The rectifying and filtering circuit rectifies and filters outputs of the resonant and transforming circuit, and generates a driving voltage accordingly. The over-current protecting circuit is coupled to the power terminal and crosses over the resonant capacitor to form a clamp path. The over-current protecting circuit detects a current flowing through the resonant and transforming circuit or a load and determines whether to conduct/cut off the clamp path according to the detection result to limit a cross voltage of the resonant capacitor within a first voltage range.