In order to expand the detection range， the 2 μm band laser generated by thulium-doped fiber laser is used to conduct intracavity gas sensing of water vapor in human breathing gas. Firstly， the direct absorption gas sensing technology is analyzed theoretically. Secondly， the characteristics of thulium-doped fiber pumped by a 1 570 nm laser are studied， and the spontaneous emission spectrum is mainly distributed in the band of 1.85~2.05 μm. Then， an all-fiber thulium-doped fiber ring laser is built， further using a tunable filter， the wavelength output in the range of 1 927.5~1 985 nm is achieved and the laser linewidth of 0.05 nm is obtained， which shows the advantages of single longitudinal mode， narrow linewidth and stable output. Finally， combined with wavelength sweep technique， the spectral scanning of water vapor in the range of 1928~1938 nm is realized and eight absorption lines are resolved， which are consistent with the simulated spectrum based on the HITRAN spectrum database. The absolute error of wavelength positioning is less than 0.03 nm. The results show that the intracavity gas sensing system is suitable for 2 μm band gas sensing detection.