An amplifying circuit and an associated linearity improving method are provided to correct the AM to PM distortion of an amplifier, thereby improving the amplifier linearity. The amplifying circuit includes an amplifier and a correcting unit. The amplifier has a non-linear input capacitor. The correcting unit generates a correction signal according to an input signal of the amplifier, and performs an AM to PM correction according to the correction signal, thereby making the amplifier have an approximately linear equivalent input capacitor.