A multi-wavelength linear cavity thulium-doped fiber laser based on fiber Sagnac loop mirror is studied. A 1.5 m long double-clad thulium-doped fiber is used as the gain medium, and a 793 nm laser diode is used as the pump source. The fiber Sagnac loop mirror and the fiber ring mirror constitute the laser resonant cavity. By increasing the pump power and adjusting the polarization controller, laser outputs with 1~7 wavelengths are realized within the spectral range between 1 949 and 1 976 nm. The output power is on the order of milliwatts, and the optical signal-to-noise ratio reaches 40~50 dB.