The seasonal distribution of the sea surface salinity (SSS) in the East China Sea (ECS) during 1990--2002 is examined based on a historical data set from the World Ocean Database 2001 (WOD2001), the data available from the Japan Oceanographic Data Center (JODC), and the cruise observations conducted by China during 1997--2002. The SSS data set has been mapped into a regular 0.25¿ by 0.25¿ grid for the study. It is shown that the largest SSS variation is at the area where the Changjiang River Diluted Water (CRDW) changes its flow directions due to the southerly wind in summer and northerly wind in fall. The variation is further complemented by the Changjiang River discharge that reaches its maximum in summer and minimum in winter. It is also shown that the seasonal variation of the evaporation minus precipitation (E-P) has a secondary effect on the SSS distribution.