Abstract: The spatial and temporal distributions of environmental ecological parameters (including salinity, nutrients and chlorophyll a) were investigated in the Guanghai bay during two cruises in October 2014 (autumn) and April 2015 (spring). The state of eutrophication in this region were evaluated by eutrophication index (EI) and nutrient quality state index (NQI). The relationships among nutrients, the N/P ratio, phytoplankton biomass and population structure were discussed in order to provide an improved understanding of the influence of environmental factors on coastal eutrophication. The spatial distributions of nutrients displayed a declining trend from the coastal estuary toward the marine environment in spring. In autumn, the concentrations of nutrients were higher in the northwestern regions, compared with those in the southeastern area. These findings suggest an extremely dynamic cycling of nutrients that responds rapidly to changes in the phytoplankton biomass and community structure. Eutrophication occurred in Guanghai bay during spring and autumn, with a decreasing gradient from inshore to offshore sites. Eutrophication situation worsened obviously from spring to autumn.