The existence of flood surge and high suspended sediment concentration in the flood surge zone has long been discerned,bur its significance in the development of tidal flat and its mechanism are not very clear yet.This Paper deals with the flood surge process over muddy tidal flat along the Shanghai Coast, proposes a qualitative model to explain the nature of the phenomenon of hign suspended sediment concentration in the flood surge zone and the mechanism of flood Surge whose intrinsic quality is that there exists a current accelerating effect which last scores of minutes along the tidal flat in flood surge zone,and discusses its importance to the development of muddy tidal flat along Shang coast Finally,a new definition of flood surge is put forward.