Jellyfish blooms had been frequently occurred around the globe in recent decades, which had been extensively managed due to their serious ecological and socio-economic damage. However, the environment-friendly methods by eliminating polyps to early control the outbreaks of medusae from source had been rarely involved despite exiting some physical and chemical manners currently. A new approach to photo-catalytically inactivate polyps of Aurelia coerulea, was thus explored by synthesizing the P-g-C3N4 photocatalysts modified with trace Ag2O in this study. During the photocatalytic reaction, up to 84.6 % of polyps died under the weak visible light similar to their wild habitats by using the Ag2O@P-g-C3N4 composite with the Ag content of 1.11 %. This prepared photocatalyst exhibited the high electron density and photo-stability, which enhanced the harvest of visible light and decreased the recombination of photo-generated electrons and holes. The scavenger experiments and ESR results suggested that the OH and O2- were the major photo-generated reactive radicals causing the mortality of polyps. They could bring about some significant abnormal changes not only in the antioxidant enzyme activity, but also in transcriptional expression involved in permeability of the cell membrane, immune stress response and asexual reproduction for polyps. Under the attack of these radicals, polyps thus suffered severe tissue damage and eventually died. This study was the first to propose the application of photocatalytic reaction to control jellyfish blooms by inactivating their polyps and revealed the potential photocatalytic lethal mechanisms, providing new insights into solving marine ecological disasters.