The present invention is to provide an automatic gain-controlled optical fiber amplifier, comprising: a first optical branch for branching a portion of an optical signal inputted into the optical fiber amplifier; a second optical branch for branching a portion of an optical signal outputted from the optical fiber amplifier; an optical distributor for receiving the optical signal of an input side branched partially by the first optical branch and for outputting it separately; a first wavelength selector for receiving the optical signal of a one side distributed by the optical distributor and for selecting a predetermined wavelength optical signal; a second wavelength selector for receiving the optical signal of an output side branched partially by the second optical branch and for selecting the predetermined wavelength optical signal; a signal processor for receiving the optical signal of a second side distributed by the optical distributor and the predetermined wavelength optical signal selected by the first and second optical wavelength selector and for measuring a total power of an input signal and a number of input channels; and a controller for generating a control signal according to the total power of the input signal and the number of input channels measured by the signal processor.