Based on the surveyed data of water quality and phytoplankton in Hangzhou Bay , the eutrophication situation and the phytoplankton diversity etc. in the bay were analyzed by nutrient status index , diversity index and similitude index in this paper. The results show as follows. 1) The level of eutrophication decreased from the inshore to the offshore in the bay and the percentage of COD , DIN and DIP in the nutrient status index accounted for 64. 3 % , 34. 6 % and 1. 1 % respectively. 2) The distribution of diversity index of phytoplankton showed the characteristic of decreasing , increasing and then re2decreasing from the northwest to the southeast , and the axis of the lowest index was located near the A -A’line between 121°30′E, 30°00′N and 122°30′E, 31°00′N in the bay. 3) The diversity index of phytoplankton was closely related to the nutrient status index , and the offshore area was more likely to become the ”danger area”, which may result in red tide bloom because of their negative correlation in that area. 4) According to the growth environment for phytoplankton , Hangzhou Bay can be divided into inshore area and offshore area based on the A -A’line , and the inshore area was worse than the offshore area for phytoplankton growth. 5) According to the similitude index , phytoplankton community in Hangzhou Bay can be divided into inshore phytoplankton community and offshore phytoplankton community , and the boundary of their geographic distribution was the same as above.