Abstract: The effects of UV-B (280nm-320nm) on the dry weight, PSⅡ activity (Fv/Fm), pigment contents, reactive oxygen species (ROS) accumulation and antioxidant system activities of Haematococcus pluvialis were investigated, which were preliminarily seeking to the feasibility of enhancing the astaxanthin biosyntheses by UV-B irradiation. It was found that UV-B significantly inhibited photosynthetic activity and the increase of biomass of H. pluvialis. UV-B also changed the contents of cellular pigments. Its effects on carotenoid and astaxanthin were stronger than on chlorophyll. UV-B increased the cellular carotenoid and astaxanthin concentrations significantly after 12h- irradiance, whereas the cellular chlorophyll concentrations were indifferent. Then the cellular pigment contents of cells under 0.1 and 0.3 W/m2 UV-B irradiance kept increasing. Simultaneously, the pigment concentrations of 0.5 W/m2 UV-B began to decrease and they were indifferent with control at 24h. A negative correlation was found between the cellular pigment contents and UV-B intensities after a certain time of UV-B treatment. Although 0.1 and 0.3 W/m2 UV-B could induce the astaxanthin accumulation in H. pluvialis cells, the total amounts of astaxanthin were lower than control due to the inhibition of growth. The antioxidant enzymes (CAT and SOD) of H. pluvialis were sensitive to UV-B and they declined as exposure time extended. However, ROS contents did not rise under UV-B. Astaxanthin was a kind of antioxidant and the ROS content was correlated negatively with astaxanthin. It was possible that H. pluvialis eliminated ROS not by increased antioxidant enzyme activity but by astaxanthin under enhanced UV-B condition. In conclusion, UV-B could affect a series of metabolism activities of H. pluvialis and its effects on astaxanthin production depended on the intensity of UV-B. H. pluvialis used astaxanthin to eliminate the harmful ROS and this might be the most important UV-protective strategy which protects the cells from damage of ROS.