The invention provides a power amplifier circuit capable of adjusting gain dynamically. The power amplifier circuit comprises a power supply unit configured to provide a power supply signal; an input power detection unit for receiving at least one input signal and the power supply signal, detecting the power of the input signal to generate a detection signal, and pulling up or down a bias signal by the detection signal; a power amplifier unit for receiving the input signal and the bias signal, adjusting the gain by the controlling of the bias signal, and amplifying the input signal by the adjusted gain to output at least one output signal. Therefore, the gain of power amplifier circuit will be adjusted dynamically by detecting the power of the input signal, so that the output signal conforming to the actual power may be outputted.