The relationships between temporal dynamics of phytoplankton density and nutrients (NO3-, NH4+, PO4(3-), SiO3(2-), Fe) contents in the water body at Aotou waters of Daya Bay during red tide were comprehensively analysed based on the fixed position investigation of red tide in the summer of 2000 and the several years investigation data by using grey linear regression model. The relationships between phytoplankton cell density and chlorophyll a content were also analysed. The results showed that the predicted values were well consistent with the measurement values, and their correlation coefficients were between 0.51-0.83. Red tide might break out if chlorophyll a contents in water body were larger than 5.8 micrograms.dm-3. The study could provide a simple effective method to forecast red tide by sampling water samples to analyse chlorophyll a concentration, or using SeaWiFS data, and then, to calculating phytoplankton density. In addition, the primary productivity of the water areas was controlled by phosphorus.