An amplifier, a dynamic-bias generation device of the amplifier and a dynamic-bias method of the amplifier are provided. The dynamic-bias generation device comprises an input stage, an output stage, a detection unit, a dynamic-bias generation unit and a switch unit. The input stage having a plurality of bias terminals converts an input signal of the amplifier into at least one current signal according to the voltage of the bias terminals. The output stage receives and converts the current signal into an output signal of the amplifier. The detection unit detects the current signal. The dynamic-bias generation unit generates a plurality of bias voltages according to the current signal. The switch unit dynamically determines the connection relations between the bias voltages of the dynamic-bias generation unit and the bias terminals of the input stage according to the detection result of the detection unit.