Abstract: The water quality parameters such as pH,DO,COD,PO₄-P,SiO₃-Si,NO₂-N,NO₃-N in the Haikou Bay were monitored respectively before and after No.9618 typhoon occurring on September.18,1996.Based on the statistics of typhoon in the Haikou Bay and numerical calculation of stormy current,the mechanism of water quality variation caused by typhoon was discussed.The studies are as follows:The typhoons influenced on the Haikou Bay usually appear between July and November,most usually between August and October.The monitoring result before typhoon is different from that after typhoon.The stormy wave and windstorm current stir up the sediment in nearshore sea bottom and make bottom water mix with surface eater strongly,specially windstorm current with strong velocity in the top bay stirs up higher pollutants sediment near sea area of discharge ditch,and strong rain with typhoon carries the pollutants from land through the Nandu River to enter into the Haikou Bay,so the contents of COD,PO₄-P,N₂-N,NO₃-N,SiO₃-Si after typhoon are higher than those before typhoon.Windstorm current is violent,which made offshore high DO water exchange more frequently with inner bay water and oxygen in the air dissolve in sea water fast,so DO content after typhoon is higher than that before typhoon.This strong action of water exchange also causes pH change small before and after typhoon.