The use of high alkaline medium is a feasible way to provide carbon source and prevent biological contamination for the outdoor cultivation of alkaliphilic microalgae and cyanobacteria. A novel cyanobacterial strain was isolated from the open pond of a marine green alga ( Picochlorum sp. SCSIO-45015, Sanya, Hainan) and identified as Cyanobacterium sp. SCSIO-45682. The effects of initial sodium bicarbonate (NaHCO 3 ) concentrations on the growth and biochemical composition of Cyanobacterium sp. SCSIO-45682 were investigated. The results demonstrated that Cyanobacterium sp. SCSIO-45682 had good adaptation to 16.8-g/L NaHCO 3 (the same concentration of NaHCO 3 used in Zarrouk medium for Spirulina ). Moreover, the yields of biomass, polysaccharide, chlorophyll a (chl a ), and phycocyanin increased under high NaHCO 3 concentrations. The maximum final biomass concentration of 2.5 g/L was observed at 8.4-g/L NaHCO 3 , while the highest intracellular total saccharide content of 49.2％ of dry weight (DW) and exopolysaccharide (EPS) concentration of 93 mg/L were achieved at the NaHCO 3 concentration of 16.8 g/L. The crude protein content declined under high NaHCO 3 concentrations, which provide a possible explanation for the accumulation of polysaccharide. This study shows a good potential of alkaliphilic Cyanobacterium sp. SCSIO-45682 as a polysaccharide feedstock.