Variations of environmental factors during two Phaeocystis globosa blooms were studied and the causes for the decay were discussed. The results showed that the algal colloid vesicles and cell density were 200-3000 ind·m^-3 and 1.8×10⁷-1.5×10⁸ cells·L^-1 respectively during the peaks of the blooms, and decreased to 1-200 ind·m^-3 and 1.0×10⁶-8.3×10⁷ cells·L^-1 respectively during the decaying period. Colloid vesicles and cell density markedly decreased during the decaying period and the environmental factors changed significantly during the decaying stage. Water temperature, pH, DO and COD contents decreased significantly, while the concentrations of DIN and reactive phosphate were low and the N/P ratio was under a trend of increase during the decay of blooms. It is speculated that phosphate limitation, low temperature, and high salinity were the major causes for the decay of P. globosa bloom. This study provided some clues for the construction of early warning system and the surveillance of red tide in future.