Example fiber amplifiers and gain adjustment methods for the fiber amplifiers are described. One example fiber amplifier includes a first power amplifier, a wavelength level adjuster, and a controller, where the first power amplifier and the wavelength level adjuster are sequentially connected. The controller includes a first input end and a control output end. The first input end is configured to receive an input optical signal of the fiber amplifier, and the control output end is configured to output a first amplification control signal to the first power amplifier, and output an adjustment control signal to the wavelength level adjuster. The wavelength level adjuster is configured to perform power adjustment on each wavelength based on the adjustment control signal.