In this paper, a ring cavity Tm-doped fiber laser with both single longitudinal mode and ultra narrow linewidth is proposed and demonstrated. An unpumped polarization maintaining Tm-doped fiber (PM-TDF) and a narrowband F-P filter are combined to realize the single longitudinal mode lasing operation and ultra narrow linewidth. Experimental results show that the proposed laser is stably operating at central wavelength of 1942.03 nm and the optical signal-to-noise ratio is 63 dB at room temperature. Through 100 minutes of continuous measurement, the output power fluctuation is less than 0.62 dB and the center wavelength shift is less than the minimum resolution of the spectrometer of 0.05 nm. This indicates that it has good stability for a certain period of time. Through the linewidth measurement based on frequency noise, linewidth of the laser under the measuring time of 0.01 s and 0.1 s are 300 Hz and 3 kHz, respectively. The proposal will be applied significantly in the field where the longitudinal mode and linewidth specificity of a laser are strictly required for 2 μm wave band.