Abstract: Outbreak of the crown-of-thorns starfish (Acanthaster cf. solaris) have led to severe degradation of coral reefs in the South China Sea. Understanding the environmental drivers behind their population increase is of great theoretical and practical importance. In July 2022, we investigated the population dynamics of A. cf. solaris and associated environmental variables in Zhongsha Atoll. The average density of A. cf. solaris was 65 ind/hm², with the highest density (633 ind/hm²) at Station 10 in the southeastern reef area of Zhongsha Atoll. Environmental DNA (eDNA) and quantitative real-time PCR techniques were used to detect crown-of-thorns starfish mitochondrial cytochrome c oxidase subunit Ⅰ (CoTS-mtCOⅠ) gene fragments of the starfish in surface seawater. The target gene was detected at all stations, with concentrations ranged from 1.56×10⁵ copies/m³ to 7.38×10⁶ copies/m³, and the highest concentration found at the eastern reef area of Zhongsha Atoll. A significant positive correlation was observed between CoTS-mtCOⅠ concentrations and ammonium nitrogen levels in bottom waters (r=0.510, P＜0.05). These findings indicate a wide distribution of the starfish in Zhongsha Atoll. The outbreak occurred in the eastern reef area in July 2022 was likely caused by elevated nutrient levels, particularly ammonium nitrogen.