The invention relates to a micro-tapered fiber for producing an ultra-narrow linewidth fiber laser. The micro-tapered fiber is produced by ordinary single-mode fibers through a tapering process. A plurality of tapering regions are formed on the ordinary single-mode fibers through multiple tapering operations, and the whole micro-tapered fiber is connected into the laser, so that the length of a resonant cavity of the laser is more than or equal to 80m; the tapering regions on the ordinary single-mode fibers can effectively suppress the Brillouin gain and ensure the stability of single-frequency operation of the laser, and meanwhile, the large-length resonant cavity can meet the requirement of vast accumulation of Rayleigh gain and improve the signal strength. The micro-tapered fiber provided by the invention has the beneficial technical effects that the single-frequency operation of the laser is ensured, and meanwhile, the accumulation of Rayleigh gain can be effectively realized, and the output signal strength is improved; the tuning range is unrelated to the matching of the pumping wavelength and the length of the laser cavity, and additional linewidth compression is not needed; and the structure is simple, the implementation is easy, and the fiber is a novel narrow-linewidth light source.