World-largest macroalgal blooms (MAB) caused by Ulva prolifera outbreak every summer in the Yellow Sea since 2007, which bring potential damage to regional aquatic environment and economic. Evaluation of the evolution of historic eutrophication status is of importance for exploring the reasons of the blooms and further management. In this study, a novel nutrient pollution index weighted by area (AWCPI-NP) and ocean color data were proposed to assess temporal changes in eutrophication status in the in the Yellow Sea basin, and significant shifts were observed from the pre-MAB phase (2001-2006) to MAB phase (2007-2012): the average AWCPI-NP was higher than that in the pre-MAB phase (2001-2006) by about 45%; Chlorophyll a concentration (Chl-a), an indicator of eutrophication, increased by about 15% in the bloom region. Meanwhile, in the Jiangsu Shoal, the origin place of the floating macroaglae, the annual in-situ nutrient concentrations increased rapidly from 2000 to 2011. This new finding of progressive eutrophication behind the MAB implies that the large scale of MAB in the Yellow Sea might be primarily linked to eutrophication. Accordingly, we suggest policymakers include this new knowledge when countermeasures are considered to control this hazard.