Laser heterodyne spectroscopy has been widely used in greenhouse gas detection and laser atmospheric transmission in recent years, because of its high spectral resolution, simple structure and low cost. A laser heterodyne spectrum detection system using 3.66 μm DFB laser as local oscillator source is designed. The real-time and accurate measurements of water vapor absorption spectrum in the whole atmosphere are also realized. Furthermore, the signal-to-noise ratio after 12 averaging is 160.4, and the spectral resolution is 0.009 cm^-1. The average values of water vapor column concentration in Hefei on May 22 and 23 were 1549 ppmv and 1730 ppmv respectively. The analysis of correlationship indicated that Compared with EM27/SUN measured by Fourier Transform Spectrometer, the correlation is 0.895 and 0.819 respectively, and the deviation is 14.2% and 11.2% respectively. With the design and research of the system, the real-time measurements of the solar absorption spectrum of water vapor in the atmosphere and the precise inversions of water vapor column density are realized, which lays a foundation for more accurate detection and concentration inversion of the absorption spectrum of water vapor and its isotope HDO.