Abstract: Oncomelania diffusion is the major way of spreading s chistosomiasis. Based on the relation between environmental factors and oncomelania subsist, the influences of environmental elements such as temperature, latitude, elevation, water level, soil vegetation etc. on oncomelania subsist, and of environmental changes and human activities on the spreading of oncomelanias, are analyzed. A viewpoint is presented that eco - propagation is the fundamental power of oncomelanias spread. The study results show that the oncomelanias spread is a slow progress, and that, with a short time (a few years or decades), the flood is the most important factor to influence oncomelanias spread. Flood influences have two effects: on one hand, when flood water level raises, oncomelanias proportion is restrained; on the other hand, flood can spread spiral shells to areas without oncomelanias. So the oncomelania quantity reduces first but increases following the flood.