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This paper proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. For the first time, the TCR-CC filter is used to select a single mode from dense longitudinal modes. Experimental results show that laser in the wavelength of 1941.28 nm can maintain exceptional stability with an optical signal-to-noise ratio of 74.1 dB. The measured maximum wavelength drift and power fluctuation are 0.01 nm and 0.45 dB, respectively. Meanwhile, the measured linewidth of the laser is 910 Hz, and the relative intensity noise is below −125.82 dB/Hz above 2 MHz frequencies.