An output amplifier of a source driver and a control method thereof are provided. The output amplifier includes a first switch circuit, a second switch circuit, a third switch circuit, a first input stage circuit, a second input stage circuit, a first active load circuit, a second active load circuit, a first positive output stage circuit, a first negative output stage circuit, a second positive output stage circuit and a second negative output stage circuit. In a first display stage of the control method, the first switch circuit outputs a positive reference signal through a first output terminal and outputs a negative reference signal through a second output terminal. In a second display stage of the control method, the signals outputted by the first output terminal and the second output terminal are exchanged.