The invention discloses a predicting method and device of discharge time of magnetic device, which is characterized by the following: a conversion circuit generates a conversion signal and a subsidiary signal; the conversion signal is used to adjust a conversion adjustor; the subsidiary signal is used to control a synchronous rectifier; an estimating circuit responses the input signal and conversion signal to generate a timing signal; the input signal is relative to an input voltage of conversion adjustor; the timing signal cuts the synchronous rectifier, which inhibits the reversing current under light-load and non-load condition.