The invention relates to a hold time prolonging circuit used for a power convertor. In a scheme provided by the invention, a power system is provided with the power convertor and the hold time prolonging circuit; the power convertor is suitable for being used for converting a first input voltage at the input position of the power convertor into a first output voltage at the output position of the power convertor, and the hold time prolonging circuit comprises a boosting level and a depressurizing level which are coupled with each other through a first energy storage capacitor; the input of the boosting level is coupled with the input of the power convertor, the output of the boosting level is coupled to the input of the depressurizing level, and the output of the depressurizing level is coupled to the input of the power convertor; the boosting level is suitable for being used for changing the first input voltage of the power convertor into a second output voltage, and the depressurizing level is suitable for being used for changing the second output voltage of the boosting level into the first input voltage of the power convertor; and the boosting level can be used for maintaining the second output voltage on a first energy storage capacitor, and the depressurizing level is started when the value of the first input voltage of the power convertor is lower than a predetermined value.