The present invention discloses a source driver and a method for restraining noise output by a source driver during power on/off of a power supply. The source driver includes a multiplexer, at least two channels and at least two output pads. The channels are connected to the output pads via the multiplexer. The source driver is powered by a first supply voltage from the power supply. The two output pads are connected via a charge sharing switch. The method comprises the following steps. First, determine whether the first supply voltage is insufficient, and if yes, perform the following steps. Turn off the charge sharing switch. Then, disconnect the channels from the output pads by the multiplexer.