An LLC resonant converter with variable resonant tank includes a switching circuit for converting a DC voltage into switching signal, a variable resonant tank coupled to the switching circuit for receiving the switching signal to provide a primary current, a transformer circuit having a primary and a secondary side winding, and a rectifying/filtering circuit to rectifying and filtering a secondary current. The variable resonant tank is coupled between the switching circuit and the transformer circuit, which includes a variable resonant inductor, a magnetizing inductance and a resonant capacitor coupled in series for dynamically adjusting the gain curve of the LLC resonant converter according to the demand of the output current.