The invention relates to a control device and a control method which are applied to a flyback converter. The flyback converter comprises an auxiliary switch. The control device comprises a conductiontime setting unit and a conduction time control unit, wherein the conduction time setting unit is used for setting a conduction time threshold according to an excitation negative current reference value and the output voltage of a flyback converter; and the conduction time control unit is used for outputting a control signal, controlling the conduction of the auxiliary switch, and turning off theauxiliary switch when the conduction time of the auxiliary switch reaches a conduction time threshold. In the invention, the zero-voltage switching of the primary side switch tube of the flyback converter under the different output voltages can be realized.