Abstract: Water environment monitoring was conducted in the cage fish farm area and its vicinity (as control area) in Dapeng Cove in Daya Bay from 2001 to 2002 on a seasonal basis. The results showed that water quality in the area was in relatively good condition, except the dissolved oxygen (DO) in summer and sulfide in spring both exceeded the criteria of the second class of Sea Water Quality Standards for fisheries in China with exceeding rate of 62.5%, the other water environmental factors were general within the criteria. DO and chlorophyll a (Chl a) in the fish farm were both lower than that in control area significantly (t-test, P0.05), while the concentration of NH4-N and PO4-P in the fish farm were general higher than that in control area, and other environmental factors did not display evident differences between fish farm and control area. The seasonal variation of CODMn, BOD5 and sulfide mainly related with the seasonal characteristics of fish farming. The seasonal variation of seawater temperature, salinity, transparency, suspended solid, DO and pH were mainly determined by the subtropic monsoons climate in Daya Bay; the seasonal variation of nutrients such as NO2-N, NO3-N, DIN, DIP and SiO4-Si were mainly regulated by the rainfall and phytoplankton status, and the seasonal variation of Chl a was mainly related with temperature and N:P ratio, so there were no evident relationship between these factors and cage fish farming.