Abstract: The foundation play a significant role in the reliability of debris flow grille dam. Compared with the exposed structures, foundation scour under debris flows is more important. On the basis of a new proposed debris flow grille dam with ground anchor and counterfort, the foundation scour is considered as two stages, that is, the initial and the mid-late. In the initial stage, surface scour is primary, and in the mid-late stage the changing and progressing of scour pit is important. The formula for calculating the scour depth of foundation of grille dam is deduced based on the energy method. The scour depth is analyzed under different flow depths based on an example. The results obtained from the above formula are compared with those obtained from the other formula. The results indicate that the scour depth by the deduced formula is larger than that by the others, and the initial scour should be considered carefully in the future design.