Ulva prolifera (Müller) J. Agardh is the main causative species of the 2008 Yellow Sea green tide incident. We investigated the influences of diluted seawater on the vegetative growth and reproductive cell formation of the alga. The thalli that were cultivated under low salinities (10‰ and 20‰) and low pH values (pH around 7.0) showed obvious and steady biomass gain, while those cultivated under high salinities (40‰) and relatively higher pH conditions (pH around 8.0) manifested significant biomass loss. The trend was, however, completely the opposite for reproductive cell formation and there were indications that enrichment, over very wide concentration ranges of both nitrogen and phosphate, could significantly promote vegetative growth. Results also indicated that relatively low salinity and low pH regimes boosted vegetative growth but were unfavorable for reproductive cell formation and vice versa. Based on these results, the possible origin and development mechanisms of the green tide event are discussed. Eutrophication in the Changjiang River estuary and adjacent sea areas, as well as extremely high freshwater inflows before, and during, the flood of 2007 – due to the full operation of large-scale water facilities in the area – may have both played an important role in the formation and development of the green tide event.