Abstract: Phaeodactylum tricornutum ICE-H, a diatom isolated from Antarctic sea ice, was subjected to a certain intensity of UVB (70 μw/cm²) radiation for 0−7 days, and the changes in its physiology and biochemistry were studied to understand the effect of UVB radiation on its growth and biological composition. The results showed that: UVB stress decreased the growth rate of Phaeodactylum tricornutum ICE-H, decreased dry weight and total protein content of it, and increased total lipid content. Total monounsaturated fatty acid (MUFA) decreased, and total polyunsaturated fatty acid (PUFA) increased. The content of photosynthetic pigments chlorophyll a, chlorophyll b, and total chlorophyll all decreased. The activity of antioxidant system was correspondingly obvious, the generation rate of reactive oxygen species (ROS) increased, malonaldehyde (MDA) produced in a large amount and superoxide dismutase (SOD) accumulated in large quantities. The results of this study can provide further understanding of the mechanism of Phaeodactylum tricornutum ICE-H in response to UVB radiation from the physiological and biochemical level, and provide enlightenment for the response of Antarctic ecosystems to UVB radiation.