The intertidal red alga Porphyra yezoensis is an important marine crop which has a high stress tolerance. The salinity of the remaining water on the surface of the P. yezoensis thalli increases significantly during emersion, however, thalli can survive this severe osmotic dehydration. In order to investigate the mechanism of high salinity tolerance in P.yezoensis, the effects of high salinity on S-Adenosylmethionine synthetase (designated as PySAMS) gene expression and photosynthetic characteristics in P. yezoensis were determined. At first the thalli of P. yezoensis were subjected to seawater with high salinities (40, 50, 60, 70 or 80 salinity) for 1h. Then the oxygen evolution and consumption in cellular processes of photosynthesis and respiration were measured using the Oxygraph oxygen electrode system. And the relative mRNA expression levels of PySAMS gene were investigated using the real-time PCR. The results show that both photosynthesis and respiration were significantly influenced by seawater with 50, 60, 70 or 80 salinity in P. yezoensis thalli. The higher was the salinity of seawater, the lower was the oxygen evolution rate, and the lower was the oxygen consumption rate. The PySAMS gene expression was also influenced by seawater with high salinities significantly in P. yezoensis thalli. Seawater with 40 and 50 salinity induced the expression of PySAMS gene. However sawater with 60, 70 and 80 salinity repressed the expression of PySAMS gene. Based upon these we speculated that P. yezoensis thalli reduce the metabolism rate gradually to survive the unsuitable environmental condition when thalli suffer high salinity stress.