Abstract: The feature of humidity detector to measure the content of water in pure helium of main loop of reactor was discussed, and the circumstances using Al₂O₃ moisture sensor to measure the content of water in helium on-line were introduced. During those few years, the dew-point meter has sufficed to measure the content of water in the helium gas during the running of the reactor. On the other hand, excursion can happen when the instrument running. Demarcate must be made termly to improve the veracity of the meter.