The invention discloses a compensation device for a power amplifier and a related method. The method for determining a pre-distortion value of the power amplifier comprises the following steps of: providing a first power input signal to make the power amplifier generate a first power output signal; receiving a first receiving signal which is acquired according to the first power output signal; detecting the predetermined gain of the power amplifier by using the first power input signal and the first receiving signal; providing a second power input signal to make the power amplifier generate a second power output signal; receiving a second receiving signal which is acquired according to the second power output signal; and determining a pre-distortion amplitude value and a pre-distortion phase value of the power amplifier by using the predetermined gain, the second power input signal and the second receiving signal.