Abstract: Radiation effects of lowlevel radioactive pollutants from coastal nuclear power plants on phytoplankton and its ecosystems have attracted a growing and widely concern. In order to investigate the radiation effects of artificial radionuclides on marine planktonic algae, the morphology, growth rate and chlorophyll fluorescence characteristics of Phaeodactylum tricornutum were studied under exposure of ¹³⁷Cs radiation. The results showed that the average growth rate of P. tricornutum in the ¹³⁷Cs treatment group with radioactivity of 1 000 Bq/dm³, CsCl control group and blank group were 0.600/d, 0.600/d and 0.610/d, respectively. Compared with the stable isotope control group and blank group with CsCl added, the difference among the groups was not significant (p>005). The values of F_o and F_m increased with the culture time under ¹³⁷Cs exposure. The ratios of F_v/F_m,F_v/F_o and NPQ tended to be stable after rising, and their stability ranges were 0.5～0.6, 1.0～1.4 and 0.7～1.1, respectively;Q_p trended to decrease until it stabilized in range of 0.5～0.6. There was no significant difference in growth rate and chlorophyll fluorescence characteristics between the treatment group and the control group. The research results could provide a theoretical basis and practice references for the prevention and governance of marine radioactive pollution.