Coral colonies of Porites lutea and P.lobata were collected from the east offshore of Hainan Island, South China Sea. Oxygen isotope ratios, 18O/16O, were analyzed along the major axis of growth at a resolution of 25-28 samples within an annual band from three colonies in order to study the relation between coral oxygen isotopic composition and the monsoonal climate in seasonal, inter-annual and inter-decade time scales. The results indicate that the seasonal and inter-annual variations of coral δ 18O are mainly controlled by temperature. While the coral δ 18O is mainly controlled by SST, deviations of the δ 18O from SST correlate statistically with sea surface salinity. Therefore, we suggest that the interannual variation of the annual coral δ 18O minima is mainly controlled by seawater composition. Seawater composition analysis indicates that its δ 18O and salinity are simultaneously affected by freshwater inputs primarily from precipitation, which is mainly supplied by the Asian summer monsoon. Thus seasonal alternations of the winter and summer monsoon mainly control the coral δ 18O, thereby allowing reconstruction of these monsoonal variables. These inter-decadal trends are roughly consistent with salinity changes, but not with temperature and precipitation, suggesting that seawater composition controlled by freshwater inputs from distant source areas, ratherthan from local precipitation, is responsible for the trends in seawater composition and coral δ 18O which is probably the consequence of monsoonal precipitation over the source areas of freshwater, evaporation and the water exchange between South China Sea and Pacific Ocean. Coral colonies of Porites lutea and P. lobata were collected from the east offshore of Hainan Island, South China Sea. Oxygen isotope ratios, 18O / 16O, were analyzed along the major axis of growth at a resolution of 25-28 samples within an annual band from three colonies in order to study the relation between coral oxygen isotopic composition and the monsoonal climate in seasonal, inter-annual and inter-decade time scales. The results indicate that the seasonal and inter-annual variations of coral δ 18O are mainly controlled by the temperature. While the coral δ 18O is mainly controlled by SST, deviations of the δ 18O from SST correlate statistically with the sea surface salinity. Therefore, we suggest that the interannual variation of the annual coral δ 18O minima is mainly controlled by seawater composition. Seawater composition analysis analysis that that δ 18O and salinity are simultaneously affected by freshwater inputs registered from precipitation, which is mainly supplied by the Asia Thus, seasonal alternations of the winter and summer monsoon mainly control the coral δ 18O, thereby allowing reconstruction of these monsoonal variables. These inter-decadal trends are roughly consistent with salinity changes, but not with temperature and precipitation, suggesting that seawater composition controlled by freshwater inputs from distant source areas, ratherthan from local precipitation, is responsible for the trends in seawater composition and coral δ 18O which is probably the consequence of monsoonal precipitation over the source areas of freshwater, evaporation and the water exchange between South China Sea and Pacific Ocean.