Abstract: Cracks on drainage gallery at dam block 19 were found recent years. The cause of cracks and its effects to dam safety was listed as a special itemin the second periodic daminspection. According to the results of FEM, cracks would not appear under normal conditions. It was found that the cracks appeared during construction by re- referring to construction records. The cause of cracks and its effects to damsafety were analyzed according to geological data and temperature monitoring data in construction.