In order to design a wavelength-tunable passively mode-locked thulium-doped fiber laser, the method of changing its working wavelength by adjusting the laser polarization state based on random birefringence effect of laser cavity was proposed. The principle analysis and experimental verification were carried out. The results show that, the adjustable polarization controller can achieve mode-locked pulse output at multiple central wavelengths such as 2010nm, 2019nm, 2024nm, 2050nm, and can also be tuned precisely near a single central wavelength. This kind of tunable mode-locked laser has simple structure and good tunability. It has certain reference value for light source selection in optical communication, ultrafast optics, medicine, remote sensing technology and radar.