Abstract: In order to generate the multi-wavelength fiber laser at 2.1 μm band, a tunable multi-wavelength Ho³⁺-doped ring fiber laser based on all-fiber Sagnac interferometer was designed. A 3 m long Ho³⁺-doped silica fiber was pumped by a Tm³⁺-doped fiber laser, and then the spontaneous radiation at 2.1 μm band was amplified. In the annular cavity, the Sagnac fiber interferometer consisted of an 8 m long polarization maintain fiber (PMF) and a polarization controller (PC), and the output wavelength at 2.1 μm waveband can be period tuned by the Sagnac interferometer. In experiment, a multi-wavelength laser at 2.1 μm waveband can be got, the output power can be tuned from 1 mW to 15 mW, and up to 6 wavelengths of laser output can be observed. By adjusting the PC in annular cavity, the output tunable wavelength number can be realized from 1 to 6 at 2.1 μm band.