Abstract: The accumulation of astaxanthin in Scenedesmus obliquus under stress conditions was analyzed, and the respon-ses of photosynthetic activity and morphological change of algal cells were observed in the study. Under the temperature of content of carotenoid rose up from 0.25 mg/L to 0.44 mg/L in 48 hours. The composition of individual carotenoid was iso-lated and identified by HPLC/MS analysis. The results showed cells accumulated secondary carotenoids such as echinenone, adonixanthin, canthaxanthin, adonirubin and 3'-hydroxyechinenone and so on, the ketocarotenoid astaxan-thin (3,3'-dihydroxy-β, β-carotene-4,4'-dione) was found as a final product for the synthesis of secondary carotenoid. With the accumulation of secondary carotenoids, the algal coenobium composed of 4 or 8 cells was split up into single or two cells, and the shape of cells changed into swollen and irregular contrast to their initial state. The photosynthetic activity was also influenced by the stress conditions. The photosynthetic rate decreased about 50% in the first 3 hours, and then went up from 19.54 μmol O2/mg Chla/h to 34.29 μmol O2/mg Chlα/h in the next 9 hours. From 12 hours to 48 hours, the photosynthetic rate experienced a dramatically drop and reduced to nearly 5.21μmol O2/mg Chlα/h. The respiration rate of algal cells showed an inverse trend, which increased from 18.24μmol O2/mg Chlα/h to 60.37μmol O2/mg Chlα/h in the first 24 hours although there was a fluctuation in this course, then it decreased to 38.40μmol O2/mg Chlα/h in the next 24 hours which was still more higher than that of the control group. The change of chlorophyll fluores-cence tended to be similar to that of photosynthetic rate and decreased by 63.9%. These results indicated that the S. obliquus cells could biosynthesize astaxanthin by induced conditions. The accumulation of secondary carotenoids led the changes of contents ratio for chlorophyll to carotenoid. The light inhibition, enhanced respiration rate and the damage to PS Ⅱ were all responses to the stress, which also yielded more metabolism products and reactive oxygen species which further engendered the biosynthesis of secondary carotenoids. Simultaneously, stress conditions inhibited the cell division and led the changes of cell morphology. The regulating of photosynthetic activity and carotenoids accumulation were both the phsiol-igical mechanism for algal cells to resist the inclement environmental conditions.