Quartz-enhanced photoacoustic spectroscopy (QEPAS) technology was invented lately. Therefore it’s an innovative method for trace gas detection compared with other existed technologies. In this paper, we studied the trace gas detection system based on QEPAS, and the atmospheric H2O was selected as the target analyte. In theory, the principles of laser wavelength modulation and signal harmonic detection were analyzed firstly, and the realizing solutions for the gas concentration retrieving and laser wavelength locking were obtained. Furthermore, the selection principle of absorption line for high sensitivity gas detection was discussed. In experiments, a continuous-wave distributed feedback(DFB) single mode diode laser emitting at 1.39 μm was used as the exciting source for the H2O vapor measurement. Using wavelength modulation spectroscopy and 2nd harmonic detection, the influence of laser wavelength modulation depth on QEPAS signal level was investigated, and the acoustic wave enhancement of the addition of micro-resonator in the acoustic detection module was analyzed as well. After optimization of the QEPAS system, a detection limit of 5.9 ppm for H2O vapor was obtained. We measured the H2O vapor with different concentrations, and the R-Square of 0.98 was achieved after the experimental data was linear fitted, indicated that the QEPAS system had an excellent linear response ability. Finally, continuous monitoring of atmospheric H2O concentration levels for a period of 12 hours was performed when the line locking mode was employed with the help of 3rd harmonic detection. The experimental results showed that this QEPAS scheme had a stable performance and outstanding continuous measuring capacity, and it can be widely used in high sensitivity on-line measurement for other trace gases detection fields.