Abstract: This paper analysed the measured data by piezometer tubes at the main dam foundation fault F11 of Hongmen hydropower station in aspects such as water level, gradient, potential, rope kit picture and so on, which indicated that the effect of anti- seepage curtain was declined, the F11 fault had been in erosion and its operation performance was deteriorated gradually. So that the paper put forward the advice that reinforcement grouting as soon as possible should be adopted to ensure damsafety.