A method and a device for detecting conduction mode is disclosed. The method uses an energy-storing element to connect with a synchronous rectifier and a load, and the load connects with the synchronous rectifier. Firstly, energy is discharged from the energy-storing element and applied to the load through the synchronous rectifier to periodically generate a detected voltage signal across the synchronous rectifier, and the detected voltage signal comprises at least one raising waveform, at least one horizontal waveform and at least one descending waveform. Then, an output is generated according to waveform of the detected voltage signal and either of a reference duration or a reference slope, so as to determine that the energy-storing element operates in CCM or DCM.