Abstract: In order to investigate the effects of different salinities on the growth and agar synthesis genes of marine macroalgae Gracilariopsis lemaneiformis, three different salinities (15, 25 and 35) were applied to evaluate correlations between agar content and the expression of genes related to agar synthesis including α-galactosidase (gla), galactose transferase (gat), α-1, 3-glycolipid sulfotransferase (gst), and galactose-2, 6-sulfurylases (gas). As results, under salinity 25, the growth rate of G. lemaneiformis was the highest with the RGR of 7.17%/d, which was higher than 6.27%/d of salinity 15 and 3.57%/d of salinity 35. Both adverse salinities showed significant increase in dark respiration rate and decrease in photosynthetic rate during the experimental period. After 15 days of culture, the agar content of G. lemaneiformis under low salt conditions was 9.27%, while that in high salt conditions was 8.09% and in normal salinity 25 was only 6.91%. On the 3^rd day of culture, the gene expression level and enzyme activity of galactosidase at low and high salinity was significantly higher than that of salinity 25. After 15 days of culture, the gla gene expression in salinity 25 was still the lowest one, while the GLA enzyme activity in different salinity showed no significant difference. Under low salt condition, the expression of gat gene decreased first and then increased. Under high salt condition, the expression of gat gene was always the highest one, which was 3.03 times of that in salinity 25. For three days of low salinity cultivation, the expression of gst and gas genes decreased significantly, but it recovered to normal level in a long term of 15 days. The content of galactose in high salinity was 3.27 times of that in salinity 25, and the monosaccharide contents of low salinity were relatively low, among them, the galactose content was extremely low. In conclusion, our research indicated that either too high or too low salinity would cause a certain degree of stress and increase of agar content in G. lemaneiformis, but the mechanism of promoting agar accumulation is different while α-galactosidase and galactose transferase played an important role in this process.