A power supply circuit includes a transformer, two load-monitoring circuits, a pulse-width modulation integrated circuit, and a burst mode control circuit. The first load-monitoring circuit is disposed on the primary side of the transformer for detecting a first current flowing through the primary side, thereby providing a corresponding first detecting voltage. The first load-monitoring circuit is configured to conduct or cut off the first current according to a control signal. The pulse-width modulation integrated circuit is configured to provide the control signal according to the first detecting voltage. The second load-monitoring circuit is disposed on the secondary side of the transformer for detecting a second current flowing through the secondary side, thereby providing a corresponding second detecting voltage. The burst mode control circuit is configured to adjust the value of the first detecting voltage according to the second detecting voltage.