Economic progress has been rapid around the Daya Bay region of the northern South China Sea (SCS) over recent decades. To investigate changes of Harmful Algal Blooms (HABs) caused by environmental situation in the bay area, the present study analyzed the occurrence of HAB and environmental conditions in Daya Bay using in situ and satellite remote sensing data for the period of 1983 - 2004. Only 14 HABs were observed during the first decade (1983 - 1993), while 55 HABs occurred during the second decade (1994 - 2004). The locations of HAB have extended from aquaculture areas to the entire bay area. In addition, the seasonality of HAB has changed from spring and autumn prior to 1994 to all year round post 1994. Changes in HAB frequency and location were found to be associated with variations in nutrients and contaminants in the bay area. Total inorganic nitrogen (TIN) was 0.021 mg l^-1 in 1986, but had reached 0.068 mg l^-1 by 1999 and the ratio of TIN/P increased also, with a peak value (18.28) in 1999. Organic contaminants have also been relatively high in recent years in Daya Bay. The seasonal extension of HAB was found to be connected partially with increased water temperature resulting from the thermal discharge of nuclear power stations. Yearly mean water surface temperature (WST) increased over the past two decades. Monthly mean WST also increased after the Daya Bay nuclear power station commenced operating in 1994, ranging from 0.2 to 2.7°C. There were about 14 algal species recorded to have caused HABs frequently in Daya Bay, such as Scrippsiella trochoidea. The number of HAB species increased in the second decade (1994 - 2004). A warm-water favoring species Peridinium quinquecorne caused a bloom in May 1999. This was the first recorded time of this species blooming in China. These changes are associated with increases in cage aquaculture, contaminants discharged from industrial and agricultural activities, and thermal discharge from the nuclear power stations in Daya Bay