The present invention provides a compensation circuit. The compensation circuit is suitable for a power amplifier. The compensation circuit includes a varactor, a voltage sensor and a control circuit.The varactor is coupled to an input terminal of the power amplifier. The voltage sensor is arranged for detecting an amplitude of an input signal of the power amplifier to generate a detecting result. The control circuit is coupled to the varactor and the voltage sensor, and is arranged for controlling a bias voltage of the varactor to adjust a capacitance of the varactor according to the detecting result. Accordingly, the present invention also provides a compensation method and an amplifying circuit. Therefore, linearity of the power amplifier can be improved.