The research on medium-infrared thulium-doped fiber lasers at 2 μm wavelength is widely used in the field of laser medical, eye-safe radar, non-metal material processing, and electro-optical countermeasure system, it has an irreplaceable role compared with other wavelength fiber lasers. An all-fiber wavelength-tunable passively mode-locked thulium-doped fiber laser is reported. This laser realizes picosecond pulses and tunable wavelength by using a semiconductor saturable absorber mirror and a high-birefringence fiber optical loop mirror. This high-birefringence fiber optical loop mirror consists of a 2×2 coupler with output ports spliced to a high-birefringence fiber. The tunable picosecond pulse laser output is obtained by changing the temperature of the high-birefringence fiber in a fiber optical loop mirror. The center wavelength tunable range is from 1952 to 1967 nm, the tuning width is 15 nm, the repetition rate is 29 MHz, and the shortest pulse width is 6 ps.