The seasonal characteristics of schistosomiasis trasmission are in relationship with these of cercaria shedding and of final host exposing to infectious water frequently. The rain is sufficient and snails are brisk in spring. People work busily, exposing to water, therefore, their opportunities of infection increase. In summer and autumn, people swim or fish and the chances of acute infection is the most. In flood, contacting with infectious water frequently and in long time, they have more chance.