The tunable diode laser absorption spectroscopic technique is used for the measurement of water vapor in high purity nitrogen. Firstly, in the self-designed multiple reflectors, the accurate light path values of the multiple reflectors are measured by using the method comprising of taking CH4 with a volume fraction of 1.007×10 -3 as the standard gas, the methane absorption spectrum at around 1654 nm, and the three-line simultaneous fitting of three absorption lines with the similar central frequencies (less than 0.01 cm -1) and the same low energy levels. Secondly, the laser interference background and the absorption background of ambient air in the tunable diode laser absorption spectroscopy system are studied and the accurate background of tunable diode laser absorption spectroscopy system is obtained. Finally, the water vapor concentrations in high purity nitrogen are measured by the 1854 nm tunable diode laser absorption spectroscopy system with a detection sensitivity of 1.14×10 -6. Through the strict background absorption deduction, the multi-line Voigt linear model is used to fit the water vapor spectrum without background absorption to obtain the water vapor concentration in high purity nitrogen. The results show that, the obtained maximum deviation is 10.33% compared with the national standard.