Aiming at the problem of the abrasion at the bottom sill of emergency gate of desilting outlet of Gezhouba Power Plant, we recorded the flow state of the generator set inlet and the movement of sediment and gravels in the sediment-flushing inlet while the generator sets were running by the underwater photography. By observation, the character of the flow and the law of the sediment movement were gotten, the influencing factors of the abrasion at the bottom sill were analyzed, and some basic measures for avoiding abrasion were given.