Abstract: We investigated the preliminary mechanisms of dissolved aluminum (Al³⁺) on marine phytoplankton through field and laboratory incubation experiments. Results from in situ experiments indicate that dissolved Al decreased the total abundance of micro-phytoplankton by inhibiting the growth of cyanophyta Trichodesmium sp.; the ratio of diatoms and dinoflagellates to total phytoplankton increased, leading to the change of micro-phytoplankton community structure. Whereas, the growth of Synechococcus was promoted after initial inhibition by Al, which resulted in the increasing of total abundance of pico-phytoplankton. Additionally, the stimulatory effects of Al on Synechococcus was observed under semi-continuous culture condition; especially in 20 μmol · L^-1 Al-treated group, the significant promotion of total pico-phytoplankton abundance and growth of Synechococcus were detected (P < 0.05). The growth of Synechococcus and the maximum quantum efficiency of photosynthesis II after long-term cultivation in dissolved Al were promoted by enhancing photosynthesis due to the increase of four photosynthetic pigments including phycoerythrin, phycocyanin, allophycocyanin and chlorophyll a. The higher Al conditions resulted in higher growth rate in exponential phase and biomass in stationary phases as well as intracellular organic carbon accumulation.