As one of the largest districts where the nail-snails (Oncomelania hupensis) exist and distribute in China, the Dongting lake area is also the widest regions where shchistosoma japonicum prevail seriously.Through adruous investigation in the areas where small prevailed, some conclusions are drawn (1) The nail-snails and the schistosoma japonicum resulted have been endangering human beings' health, resources and the environment.(2) The prevalence of the nail-snails is the direct result of the development of the nail-snails survival condition suitable ecological enviornment, and frequent flooding in the Dongting Lake area, which benefits the nail-snails to live and spread.Moreover, the water conservancy projects have provided the nail-snails for ways of spreading, which aggravates the spreading of the nail-snails-(3) As to methods to preventing the epedemic nail-snails, some ecological measures should be taken, i.e.to restore the ecological system on the catchment scale, decreasing the in-put discharge and deposit to the Dongting Lake; to develop intergrated ecological economy with high-effieiency, destroying the surviving condition for snails; to utilize the grassland resources reasonably, cutting off the snails spreading channels, etc.