Abstract: This paper briefly analysed the sludge from the drainage gallery, as well as some basic data related with the sludge. The genesis of the sludge was discussed. The analysis indicated that the genesis of sludge fromgallery drain hole was complicated. They could be the clay fromthe upstreamclay blanket or the secondary dam at the left side, or the filled materials from the joint fissure of the foundation rock mass. They also could be the mixture of the three materials above. The author thought that the deficiencies should be treated.