The disclosure relates to a control device, a control method of a power converter and a switching power supply using the control device. The control device of the power converter includes: a first control unit, which is coupled to the power converter to detect an output voltage of the power converter and is configured to generate a first control signal based on the output voltage; a variable resistor unit, which is connected to an output terminal of the first control unit to receive the first control signal and is configured to generate a resistance value based on the first control signal; and a second control unit, which is connected to the variable resistor unit and is configured to output a second control signal in order to control operations of a switch unit of the power converter.