A source driver and a driving method thereof are provided. The source driver is adapted to a display panel. The source driver includes an output buffer and a regulating unit. The output buffer has an input terminal and an output terminal. The input terminal of the output buffer receives a pixel signal. The output terminal of the output buffer is coupled to the display panel for outputting an output signal. The regulating unit is coupled to the output terminal of the output buffer, for providing a charging current or a discharging current to the output terminal of the output buffer according to a polarity of the pixel signal. Thereby, a slew rate of the output signal is increased.