Potamogeton crispus plays an important role in purifying water and maintaining a healthy aquatic ecosystem in some shallow lakes; however, mass mortality occurs to 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 hot research topic in the field of aquatic ecological restoration. Some researchers have reported that intense light is the key factor in the decline of P. crispus. The UV-B band in strong light is generally recognized to damage plants and animals, and can even penetrate into the water column and cause harm to aquatic organisms. Therefore experiments were conducted to determine the effect of the UV-B radiation in strong light on P. crispus decline. In this study, mature plants of P. crispus were daily exposed to different doses of UV-B radiation (50, 100, 150, and 200 μW/cm²) for 6 hours (09:00-15:00). The cumulative effective doses of UV-B radiation were, respectively, 10.8 kJ/m², 21.6 kJ/m², 32.4 kJ/m² and 43.2 kJ/m². The control group was only exposed to UV-A radiation and photosynthetically active radiation (PAR). PAR and UV-A radiation were set to the same levels in all treatment groups. The physiological characteristics of the experimental plants were monitored regularly under UV-B radiation, and physiological and biochemical indices and photosynthetic pigment contents were determined. The results show that the chlorophyll a (Chla), chlorophyll b (Chlb), total chlorophyll (Chlt), and carotenoid (CAR) contents can be improved by UV-B radiation at doses below 10.8 kJ/m², and can be inhibited in the decline stage. The Chla, Chlb, Chlt, and CAR contents were all decreased by UV-B radiation when the UV-B radiation dose was greater than 10.8 kJ/m², and the inhibition effects increased with increasing radiation dose. Catalase (CAT) and peroxidase (POD) activities were increased under short-term radiation and gradually increased with increasing radiation dose; however, these activities were inhabited with continuing exposure to radiation. The activity of superoxide dismutase (SOD) increased gradually with prolonged irradiation time, and the activity of SOD increased with increasing radiation dose. The malondialdehyde (MDA) content was increased at the beginning of the UV-B radiation and gradually increased with increasing radiation dose; however, the MDA content decreased with continuing radiation. The soluble protein (Pr) and soluble sugar (SC) contents were improved at the beginning of the UV-B radiation, and gradually increased with increasing radiation dose, but these contents decreased after long-term UV-B radiation, and the contents decreased gradually with increasing radiation dose. The results indicate that UV-B radiation may be an important factor leading to the mass mortality of P. crispus in late spring and early summer. Accordingly, we deduce that the sharp increases in ultraviolet radiation on the land surface in the late spring and early summer leads to mass mortality of wild P. crispus populations. However, further research is necessary to study whether UV-B radiation acts synergistically with other environmental factors to affect physiological activity in the decline of P. crispus.