Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a non-contact optical measurement method which is widely used in combustion diagnosis and propulsion system development. The selection of the absorption line pairs is the basis and key of gas temperature measurement using TDLAS. In the experiment of scanning wavelength-direct absorption method for measuring the temperature of H2O, the basic principles and implementation steps of spectral line selection are systematically studied, and the quantification of each index in the selection principle is analyzed. Then the absorption lines of water molecules in the range of 7000~7500cm-1 were studied and screened. Finally, the numerical simulations were used to analyze and two line pairs were selected: 7022.7091, 7444.3707 cm-1 and 7185.5962, 7444.3707 cm-1, which provided the basis for the design of TDALS temperature measurement system.