Efficient in-situ treatment of high concentrations microcystin-LR in surface waters is especially challenging. Herein, spherical floatable Z-scheme core-shell photocatalyst EP@n-C3N5/Ag2CO3 (EP@CNAC) with carbon defects was constructed using expanded perlite (EP) as floating carrier for efficient adsorption and degradation of microcystin-LR. The carbon defect sites and electron density distribution of C3N5 were investigated by DFT calculation. Benefited from synergistic effects of morphology and defect engineering, Z-scheme heterojunction, and floatability, the carrier migration kinetics and solar spectral responsiveness of EP@CNAC-3 were significantly enhanced, and 97% of microcystin-LR was degraded within 16min. The LC-MS, Fukui index, and electrostatic potential of microcystin-LR revealed that Adda side chain was cleaved by electrophilic attack of 1O2, while ring-opening reaction and decarboxylation were caused by nucleophilic attack of •O2- and strong oxidative properties of h+, respectively. These findings are expected to bring new insights into design of innovative floating photocatalysts and in-situ remediation mechanism of microcystin-LR.