Abstract: To determine the limited nutrient factors of phytoplankton growth, single and multi-enrichment experiments in suit was carried out to study the key limitation factor in outer of Daya Bay during 11~13, December, 2005. The single enrichment experiment results showed that the chl a concentration of Fe and N units had increased significantiy to 1.1562 and 0.9581 gL-1, and in multi-enrichment experiment the concentration of chl a in EFe and EP had reached a higher value to 1.16151,0.8778 gL-1, respectively. Time series of chlorophyll a concentrations were analyzed using one-way ANOVA model to evaluation the nutrient limitation patterns. N and Fe limit phytoplankton growth, according to evaluation results. Generally, Fe was considered as a promoting factor in phytoplankton growth in many researches, so N is the key factor that limits phytoplankton growth. The diatom as the dominate population, has a high N-demanded, and N-limitation open sea impacted obviously in the outer of the Bay. They co-determined the main limitation factor of the water.