The invention discloses a single-frequency single-polarization fiber laser in middle-infrared bands, in particular relates to a straight line single frequency laser. The laser comprises a pumping source, a pumping light isolator, a gain optical fiber component and a single mode polarization maintaining optical fiber, wherein the input end of the pumping light optical isolator is welded with the output end of the pumping source; the input end of the gain optical fiber component is welded with the output end of the pumping light isolator; the gain optical fiber component comprises a fiber core, a stress rod and a covering layer covered on the fiber core and the stress rod, the fiber core of the gain optical fiber component is made of germinate glass or germanium-containing silicate glass, and both the covering layer and the stress rod are made of the silicate glass; and the input end of the single mode polarization maintaining optical fiber is welded with the gain optical fiber component in a zero-degree angle shaft aligning manner. Therefore, the Ge-containing high doped gain optical fiber is used as the gain optical fiber, so that the length of a resonant cavity is shortened, the output laser linewidth is further compressed, and optical grating can be directly written on the gain optical fiber, and thus efficient laser efficiency is achieved under the condition of the ultrashort gain length, and stability of output power is kept.