Abstract: The biomass of macroalgae, macrojellyfish and backwashing in the waters near the cold source water intake area of nuclear power plant in eastern Liaodong Bay was monitored from March to November in 2021, and the change trend of macroalgae and macrojellyfish was discussed in combination with the historical data from 2018 to 2020, so as to determine the high-risk period. The monitoring results for 2021 showed that the biomass of macroalgae peaked from mid-to-late May to the end of July; the biomass of Aurelia coerulea, Nemopilema nomurai and Cyanea nozakii peaked from July to September, and their total relative biomass was lower than that of 2018－2020. The relative biomass of Aurelia coerulea and Nemopilema nomurai was lower than that of 2018－2020, but the occurrence time of Nemopilema nomurai was one month later than that of 2018－2020. The relative biomass of Cyanea nozakii was basically the same as that in 2020. There was no significant correlation between the biomass of macroalgae and macrojellyfish in the waters near the cold source water intake area and the monitoring results of backwashing, indicating that the five intercepting nets at the water intake had a significant effect.The risk assessment grades of backwash sampling were all safe. Musculus senhousei was found to be a new risk species in backwash monitoring in 2021, and the occurrence time was from October to November. Attention should be paid to its risk status in the future.