A resonant converter, a power supply and a power controlling method thereof are provided. The power supply includes a resonant converter which includes a square wave generator configured to alternately turn on and off first and second switches according to a frequency to generate a square wave, a resonant wave generator configured to generate a resonant wave corresponding to the square wave and a rectifier configured to output a voltage corresponding to the resonant wave; and a controller configured to control a frequency modulation of the resonant converter, wherein the controller includes a variable switching circuit configured to increase the frequency of the resonant converter in response to the resonant converter entering a capacitive mode.