In order to study the enzymatic mechanisms responsible for polysaccharide synthesis in Chlamydomonas sp. YB-204, the changes of polysaccharide and glycometabolism related enzymes were analyzed. Under the condition of salt stress, the activities of phosphoglucose isomerase (PGI) and malate dehydrogenase (MDH) were inhibited, but hexokinase (HK) was increased. The higher concentration of nitrogen source would inhibit the activities of glucose-6-phosphate dehydrogenase (G6PDH), PGI and MDH, and not conducive to the synthesis of polysaccharide. NaHCO₃ could promote the synthesis of polysaccharide. The activities of G6PDH, PGI and HK decreased, but the activity of MDH increased when NaHCO₃ was added. The variation trend of polysaccharide content was very different with the activities of four enzymes during the growth of YB-204. Pearson correlation analysis showed that HK, G6PDH and MDH were significantly negative correlation with polysaccharide synthesis. The correlation between PGI and polysaccharide synthesis was not significant (p>0.05). We presumed that HK, G6PDH and MDH were the key enzymes to regulate the biosynthesis of polysaccharide, and there might be more than one branch to synthetize polysaccharide in YB-204.