The invention relates to a control circuit of a resonant converter. The resonant converter is connected with a load circuit, the load circuit can produce a feedback voltage, and the control circuit comprises a voltage regulator, a mode selector, a modulator and a driver. A voltage controller calculates difference of the feedback voltage and a reference voltage and carries out proportional integral to output a regulating voltage. The mode selector selects a regulating way according to the regulating voltage, outputs changing control frequency and fixed conducting time to control voltage when the regulating way is PFM modulation and outputs fixed control frequency and changing conducting time to control the voltage when the regulating way is PWM modulation. The modulator controls the voltage to produce driving pulse according to the control frequency and the conducting time, a pulse signal with changing duty ratio and fixed frequency is produced as the driving pulse under the PWM modulation condition, and a pulse signal with changing frequency and fixed effective width is produced as the driving pulse. The driver outputs a control signal to the resonant converter according to the received driving pulse.