Abstract: After surveyed in situ in Nanhui east shoal several times,the rule of freshwater is described in this paper.The probability of freshwater occurred in wet season and dry season is from 76.98 to 82.94 percent and from 35.04 to 42.66 percent,respectively.And the causes of freshwater are discussed also.The runoff of Yangtze River is abundant,which is the basis of freshwater occurring in Nanhui east shoal.The Criolis's force is another important factor making the freshwater flow at the tide flat.The velocity and direction of wind in the dry season have important effect on freshwater resource.Northwest wind accelerates the probability of freshwater resource occurred in dry season,especially.