Abstract: Based on the data of NVAP-M and ERA-I reanalysis from 1979 to 2008, the temporal and spatial distributions of rainy-season, dry-season and annual mean precipitable water vapor (PW) were analyzed. The results show that there exist considerably clear regional (southern wet-northern dry), seasonal, and interdecadal changes of PW over Hainan Island. The value of dry-season PW over South China Sea can reach 50 mm. The water vapor amount decreases with the latitude increasing, and the low center of dry-season PW is located in Wuzhi Mountain of Hainan Island. The mean PW in rainy-season over Hainan Island is greater than in dry-season. There is a single peak in annual cycle of monthly mean PW, and the maximum value(53 mm) appears in June. There is also a decreasing tendency about PW with an average decrease of 0.53 mm/10a in recent 30 years. The water vapor fluxes are obviously different between anomalously high PW years and low PW years. Water vapor transport and moisture convergence from the north Bay of Bengal and the South China Sea to Hainan Island is the key factor for rainy-season anomalous PW over Hainan Island in dry and wet years.