The concentration of water vapor is one of the important indices during semiconductor sealing element manufacturing process, and excessive water vapor has seriously influence on the quality of semiconductor. Therefore, a real-time water vapor measurement technique with high sensitivity and high precision is crucial. In the work, a low-concentration water vapor measurement system based on off-axis integrated cavity absorption spectroscopy is developed, the reflectivity of the mirror is 0.99920, effective optical path length is 250 m, and measurement time is 0.025 s. The mode density of the resonant cavity can be improved when the light is off-axis transmitted into the laser resonant cavity, thus the signal-to-noise ratio is significantly increased. The system is utilized to investigate the absorption line of water vapor near 7036.5 cm -1, it is found that the sensitivity of the system is 7.07×10 -6 cm -1, and measurement error is less than 5%. By injecting different concentrations of water vapor into the cavity, the on-line continuous measurement performance of the system is tested, the result reveals that the system can meet the process requirement.