A high power diode-pumped Tm:YLF laser operating at 1.89 μm with FWHM line width less than 0.1 nm is introduced. The YAG Fabry-Perot etalons of 0.05 mm is inserted to polarize at Brewster angle in laser cavity, and 1.89 μm horizontal polarized light is obtained. Two Fabry-Perot etalons with 0.05 mm and 0.3 mm YAG are inserted successively for spectral narrowing and laser stabilization. Under incident pump power of 137 W, 46 W continuous output power with 1 889.3 nm output wavelength and 45.1% slope efficiency is achieved. The laser wavelength shift of only 0.12 nm with the incident pump power from 20 W to 137 W is observed. The beam quality factors at output power of 15 W and 30 W are 1.7 and 2.2 respectively.