An active start judgment circuit is electrically connected to an AC/DC transforming power supply which has at least one standby power unit to transform AC to DC in regular conditions and a main power unit to transform the AC to the DC in an ON condition for operation of an electronic equipment. The start judgment circuit bridges the standby power unit and the main power unit, and generates a reference potential based on a voltage output from the standby power unit, and gets a power signal from the standby power unit to be compared with the reference potential to output a start signal to the main power unit to transform the AC to the DC. Thus the standby power unit can actively drive the main power unit to supply DC power to activate the electronic equipment.