Phaeocystis globosa (P. globosa), a main culprit of harmful algal blooms (HABs), is highly prone to blocking the inlet filter screens of nuclear power cold sources, thus posing a significant threat to nuclear power safety. However, existing methods for P. globosa detection fail to achieve rapid and on-site monitoring of single-cell densities prior to bloom outbreaks, limiting timely defensive measures. In this study, we developed a novel biosensor platform for efficient P. globosa detection, leveraging an Exo III-assisted signal amplification strategy to significantly enhance sensitivity and selectivity. The biosensor achieved an ultra-low limit of detection (LOD) of 268.91 fg µL