The diffusion of oncomelanias by flood is a main reason of schistosomiasis propagation. Therefore it is the most essential and effective way to control diffusion of oncomelanias along with flood for preventing epidemic of schistosomiasis disease. In this paper the research on the diffusion of oncomelanias along side-weir flow is carried out. The result shows that after diffusion along side-weir flow, oncomelanias do not evenly distribute at floodplain areas, but fall in the place that flow velocity is small , i.e. in the backflow areas and at boundary places of upper reaches and lower reaches of the side-weir breach. Oncomelania’ s diffusion law provides an important theory for eliminating oncomelanias.