An LLC resonant converter includes a square wave generator having a first switch and a second switch, a resonant tank, a transformer, a synchronous rectifying (SR) unit having a first SR switch and a second SR switch, and a control unit. The control unit provides a first control signal controls the first switch, a second control signal controls the second switch, a first rectifying control signal controls the first SR switch, a second rectifying control signal controls the second SR switch. When a frequency control command is lower than a phase-shift frequency, the first control signal and the first rectifying control signal are frequency-variable and phase-shifted, and the second control signal and the second rectifying control signal are frequency-variable and phase-shifted.