In situ incubation experiments were conducted to investigate the phosphate uptake and growth variations of nano- and pico-phytoplankton controlled by the coupling between different phosphorus concentrations and different levels of irradiances in Changjiang estuary and its adjacent sea. By comparing the nutrient concentrations in incubation bottles at fixed intervals and the species numbers of both nanophytoplankton and picophytoplankton, the results were as follows: The uptake rates of phosphate increased distinctly at a high phosphate level (0.60 μmol/L) under high irradiance (100% natural irradiance), which showed that the growth of phytoplankton was strongly phosphate-limited. The cell densities of nanophytoplankton and Synechococcus sp. also increased obviously. Moreover, picoeukphytoplankton seemed to be adapted to a low phosphate level (0.25 μmol/L), which reflected that growth of different phytoplankton species had different adaptation to phosphate level. It was also possible that there was nutrient competition between nanophytoplankton and picoeukphytoplankton, when nanophytoplankton grown and became dominate species at a high phosphate level the growth of picoeukphytoplankton could be limited. Therefore, the larger phytoplankton species was in favor of a high phosphate level in the maximum turbidity zone of the Changjiang estuary. Under low irradiance (50% of nature irradiance), phosphate uptake was restrained at high phosphate levels and the growth of both nanophytoplankton and synechococcus sp. was also limited. Moreover, they grew well at intermediate phosphate levels (0.41 μmol/L) and picoeukphytoplankton grew well at a low phosphate levels. In fact, there was no such environment with a low irradiance and a low phosphate levels in the maximum turbidity zone, so that phytoplankton bloom hardly occurred there. In addition, the results also showed that the growth periods of all three kinds of phytoplankton at intermediate phosphate levels were prolonged obviously, suggesting that the limitation of phytoplankton growth mainly reflected its growth period changes. Without irradiance, the addition of phosphate didn’t affect the release rates of phosphate with lineal increase and the growth rates of phytoplankton with exponential decrease, which showed that phosphate regeneration was faster in day than in night and the irradiance was a significant factor to affect phosphorous biogeochemical cycle in the Changjiang estuary.