Abstract: In situ photosynthetic characteristics of phytoplankton were investigated using Phyto-PAM-II at 52 stations in 6 sections of the Changjiang Estuary and its adjacent sea area in July 2020. Combined with measured ecological environment data, the maximum photochemical efficiency of photosystem II (F_v/F_m), rapid light curve and slow induced curve were studied. The results showed that the photosynthetic activities of phytoplankton were more closely related to the estuary physical process in Changjiang Estuary and its adjacent sea area. There was a significant negative relationship between the photosynthetic efficiency with surface water temperature (p<0.01) in summer. The photosynthetic activities had a significant spatial difference among different featured surface water masses by salinity, i.e., Changjiang River Water, the Turbidity Maximum, the Changjiang Diluted Water and the East China Sea Water. The phytoplankton activity was highest in the Changjiang diluted waters and the maximum F_v/F_m was 0.48, while it was lower in the Changjiang estuary river mouth and the average F_v/F_m was only 0.26. The complex and changeable physical process of the estuary area created a diverse ecological environment for the phytoplankton community. The photosynthetic activity of phytoplankton community was lower in the high turbidity and low nutrient sea area, resulting in a low biomass. In the diluted water area, relatively suitable light and nutrients led to the high photosynthetic activities, supporting the high productivity and biomass of phytoplankton in this waters.