Abstract: The present invention relates to a plug-in circuit and a cooling system thereof. The cooling system includes the plug-in circuit, an external terminal and an alternating current (AC) compressor, wherein the plug-in circuit includes an AC to AC inverter and an inverter control circuit. The AC to AC inverter is coupled to and between the external terminal and the AC compressor for converting an external AC voltage received by the external terminal to an internal AC voltage to supply to the AC compressor. The inverter control circuit is coupled to the AC to AC inverter for outputting a control signal to the AC to AC inverter to control the frequency of the internal AC voltage according to an external parameter.