A diode laser (LD) clad-pumped narrow linewidth all-fiber Tm³⁺-doped fiber laser is reported with a maximal output power of 27 W at 1.947μm. By successively splicing an LD pigtail fiber, a single-mode Tm³⁺-doped fiber, and a multi-mode Tm³⁺-doped fiber, the fiber laser has 70 pm narrow linewidth output, and a high slope efficiency of nearly 47.5% with respect to the launched pump power. The high reflectivity fiber Bragg gratings, which are directly written into the single-mode Tm³⁺-doped fiber core by the 800 nm femtosecond pulsed laser, act as the high reflectivity coupler. The output laser has diffraction-limited beam quality with a factor M² of 1.29, when the output laser power is nearly 27 W.