This paper proposes and experimentally tests a stable single longitudinal mode (SLM) narrow linewidth thulium-doped fiber laser by utilizing a special subring cavity and homemade fiber Bragg grating (FBG). The subring cavity comprising three interconnected optical couplers is employed for suppressing the dense multiple longitudinal and hopping modes. The narrow bandwidth wavelength filter, which is composed of the homemade FBG, achieves stable SLM operation. The experimental result shows that the laser can obtain a stable 1940.6 nm output with an optical signal-to-noise ratio (OSNR) of 60 dB at room temperature. Its frequency noise is measured using a 3×3 optical-fiber coupler with self-homodyne technology. The linewidth, which is calculated from frequency noise, is approximately 8 kHz when measurement time is 0.05 s.