Abstract: Eutrophication is the most important environmental problem in the coastal marine pollution. Macroalgal blooms tend to have the potential for rapid absorption of nutrients and rapid proliferation. In order to investigate the physiological and biochemical responses of Ulva prolifera to nitrogen(N) and phosphorus (P) in the eutrophic seawater, the growth rate，photosynthesis and the pigments of U. prolifera were determined under different nutrients conditions and different culture densities . The experiment was divided into four treatment: the control group, N group P group, and N P group ( N, P represents adding NaNO3、Na2HPO4, respectively.). The algae were pre-cultured in four nutrition conditions for 3 d. The temperature was set as 20°C, light intensity was approximately 6000 lx, and photoperiod L: D was 12 h: 12 h with continuous aeration. Our results showed that either N or P supplies could increase the growth rate of U. prolifera, but much more significant enhancement was found in the N enrichment than P enrichment. When the original biomass was set as 0.1 g / L, the relative growth rates of the control group, P group, N group and N P group were 0.68, 0.69, 0.72 and 0.75, respectively. There was a negative relation between the relative growth rate and biomass density. The biomass doubling time significantly shortened when N and P were added. When the original biomass was 0.2 g / L, the biomass doubling time of control group, P group, N group and N P group was 1.60,1.53,1.16, and 1.03 d, respectively. The fastest biomass doubling time was found in the thalli grown at N and P enrichments together condition and 0.1 g /L initial biomass density, with the values of 0.92 d. Either N enrichment or P enrichment could increase the chlorophyll and carotenoid contents, as well as the net photosynthetic rate of U. prolifera. The maximum net photosynthetic rate and pigments contents in U. prolifera were found in the culture condition with a combination of simultaneous enrichment of N and P. There was a siganificant interaction between N and P enrichment. In summary, N, P enrichments can significantly promote the growth, increase the photosynthetic pigments and photosynthetic rate of U. prolifera, which may be an important mechanism of the bloom of U. prolifera.