Abstract: Seepage bringing out sand from structural joints of the gallery on dam block No.10~11 of Tongjiezi dam was a major problem threatened the safety operation. To ensure operation safety, during the second periodical inspection, cause reasons for seepage were analyzed and treatment scheme as well as construction technology were optimized. Water pressure test was replaced by closed water test to find leakage position. Followed, low pressure inclined-borehole grouting technology combined with rated weight grouting was adopted to stop the slurry and seal the structural joint. Then, LW/HW soluble polyurethane grouting was carried out. The construction technology solved difficulties such as difficult control of the water pressure test, large vertical deviation of structure joint, the small construction site in gallery, slurry leaking in grouting and the bad treatment effect at earlier stage. The fact that the seepage amount dropped from 180 L/min to 0 L/min proved the good effect of the treatment.