Potamogeton crispus is important in purifying water and maintaining a healthy aquatic ecosystems such as shallow lakes. However, mass mortality occurs in P. crispus populations in the late spring and early summer, which possibly results in secondary pollution. Therefore, the causes of P. crispus mass mortality have been a very important research topic in the field of water ecological restoration. The UV-B band in the strong light is generally recognized to damage plants and animals and even to penetrate into the water column and harm aquatic organisms. Some researchers have reported that intense light is the key factor in P. crispus decline. We carried out a series of experiments to find the effect of UV-B radiation in the strong light on P. crispus decline. In this study, adult plants of P. crispus are exposed to UV-B radiation in different doses (50 μW/cm², 100 μW/cm², 150 μW/cm², and 200 μW/cm²) for 6 hours (9:00-15:00) every day, and the control group was exposed only to UV-A radiation and photosynthetically active radiation (PAR). Plant growth, morphological changes (plant height, internodal length, leaf area and fresh weight), turion formation (number, weight, and morphology of turion) and the germination indexes of turion were monitored. The results show that UV-B radiation can accelerate plant decline. When the exposure dose is in excess of 150 μW/cm², the plants decline and fall within 31 days, while a few plants in the other treatment groups still grow. The tops of the plants are damaged more when they receive higher doses of UV-B radiation, so that plant height declines persistently with UV-B radiation, the internodal length is shortened by UV-B radiation, the leaf area shrinks, and the average fresh weight per plant decreases; whereas the plant height and internodal length of the control group increase till the 24^th day, the leaf area also increases till the 16^th day, and the average fresh weight per plant decreases gradually with increasing radiation doses. UV-B radiation had no effect on turion quantity in both the treatment and control groups, but the metamorphosis ratio of turion increases with the increasing radiation dose. When the exposure dose of UV-B radiation increases, the turion length increases, whereas the turion width and weight are reduced, the germination rate from the next growing season is reduced. The ratio of two seedlings that germinated from these turions decreases and the seedling growth indexes (germination rate, plant height, leaf number, leaf length, leaf width, leaf area, and so on) decrease and are significantly reduced especially when the exposure dose is in excess of 150 μW/cm². The results indicate that UV-B radiation may be an important factor leading to mass mortality of P. crispus in late spring and early summer. Accordingly, we deduce that the sharply enhanced ultraviolet radiation on the land surface in the late spring and early summer leads to mass mortality of the natural P. crispus population. However, further research is necessary to study whether UV-B radiation acts synergistically with other environmental factors on the decline of P. crispus.