The adult plants of Potamogeton cripus were exposed to UV-B radiation with different doses (50, 100, 150 and 200μW/cm²) in lab, which were checked for the degree of injury to photosystem Ⅱ in the plants by means of modulated chlorophyll fluorometer (DIVING-PAM).The results showed that the maximum leaf photosynthetic rate, light-resistant ability and light-harvesting capacity all increased with the increasing radiation dose at the initial stage.The maximum photosynthetic rate, light-resistant ability and light-harvesting capacity were promoted when the UV-B radiation dose below 100μW/cm² in the initial 16 days, and were inhibited when the dose beyond 100μW/cm² when the inhibitory effect lasted with prolonging the irradiation time.Eventually, the treatment group all declined when the dose beyond 100μW/cm², and the differences between the treatment groups were small when the dose below 100μW/cm².It indicated that the maximum plant photosynthetic rate, light-resistant ability and light-harvesting capacity were injured less when the dose below 100μW/cm², while the photosystem Ⅱ was injured by UV-B radiation when the dose beyond 100μW/cm², so that the photosynthetic capacity reduced.Because the intensity of ultraviolet radiation on land surface increased in the late spring and early summer, the degree of damage to the plants increases evidently.Thus, the results of this study indicate that UV-B radiation may be an important factor leading to mass mortality of P.crispus in late spring and early summer.