Abstract: In this paper,samples from thunder showers controled by subtropical high during the south-west summer monsoon, observed in Yongxing QXisha islands) during May, 1987, are analyzed. The pH values of all samples are about 7.0.The concentrations of chlorine and sodium ions are large, about 62.5% and 36.1% of the negative ions and positive ions respectively. The ratio of chlorine to sulfate ions is 2.3,different from that observed over land in our country. These results may represent the chemical composition of precipitation over South China Sea region for atmospheric science and environmental science.