This paper deals with the ecological mechanisms of the depth profile, the photoinhibition in wate upper layer, the seasonal and diel variation of the primary productivity of phytoplankton, based on the measurements on the primary productivity and some environmental factors, in the Miaohu Lake, a part of Lake Donghu, and the station no 1 Lake Donghu, Wuhan, during June, 1987 to May, 1988. The depth profile of primary productivity was caused by the variation of the biomass of phytoplankton with the change in depth, and also by the decrease of irridiance with the depth in the lake. The decline of the biomass of phytoplankton and the decrease in photosynthetic rate of phytoplankton under strong irridiance in water upper layer resulted in the photoinhibition of primary productivity. The photosynthetic mechanism of phytoplankton did show apparent seasonal adaption with the change of the temperature of water. This adaption was very important in the seasonal variation of primary productivity. The diel variation of phytoplankton primary productivity depended mainly on the diel fluctuation of irridiance.