A synchronous rectifier applied to a power converter includes a gate coupling effect suppressing unit. The gate coupling effect suppressing unit is used for suppressing an induced voltage coupled to a gate of a metal-oxide-semiconductor transistor coupled to a secondary side of the power converter to ensure that the metal-oxide-semiconductor transistor is turned off when the power converter operates in a start-up condition and a power switch of the power converter is turned on. When the power converter operates in the start-up condition, a driving voltage of the secondary side of the power converter for driving the synchronous rectifier is not enough to drive the synchronous rectifier yet.