To understand the displacement transformation of Shuibuya dam riverbed after overlays being pounded strongly and the disposal effect, this paper makes a G(1,1) model to forecast the displacements of reserved overlays of riverbed of Shuibuya dam by writing programs and analyzing the riverbeds transformation law and displacement mechanism. The computed results show: the gray forecast model is fit well in the analyzing and forecasting short serial data and rather high precision; at the same time, it can show the pounding effect well and reasonablenesess in economy.