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The Liaoning Hongyan River Nuclear Power Plant is located on the east coast of Liaodong Bay in the Bohai Sea, China, and the safety of the cooling water is often threatened by large-scale jellyfish outbreaks. Thus, monitoring jellyfish growth, migration, and disappearance is essential to the safety of the nuclear power plant. In this study, high-throughput sequencing was used to analyze the community characteristics of mesoplankton in the sea area at the inlet and outlet from April to September 2019. At the same time, the micro-individuals of jellyfish (including oosperm, planula, polyps, or ephyra of jellyfish) in the water were detected by real-time fluorescent quantitative PCR (qPCR). Both the maximum abundance of Aurelia coerulea and Nemopilema nomurai in July were detected by high-throughput sequencing and qPCR. Only three common taxa of plankton (Paracalanus, Calanoida, and Poaceae) were found in the waters near the inlet and outlet, and the beta diversity of plankton in the waters near the inlet and outlet was low in the same period. Based on the plankton community composition in the sea water near the inlet, it could be speculated that jellyfish outbreaks would have a significant impact on the plankton via the predation effect. The maximum mt-16S rDNA copy numbers of A. coerulea and N. nomurai in the samples from the waters of Liaodong Bay were detected in June 2019, and that of N. nomurai showed an increasing trend from north to south, with the jellyfish tending to move to the seawaters near the plant. The results indicate that the optimum monitoring period for jellyfish outbreaks in Liaodong Bay is from the end of May to the end of July. [ABSTRACT FROM AUTHOR]