The porous g-C3N4 (PCN) nanosheets are successfully synthesized and further modified with nano-sized Ag by a simple wet-chemical process. Interestingly, the Ag-modified porous g-C3N4 (Ag-PCN) nanosheets exhibit competitive fluorescence detection performance of chloride ion (Cl-) in aqueous solution. Under the optimized conditions, the concentration of Cl-could be quantitative analyzed with the Ag-PCN in a wide detection range from 0.5 mmol/L to 0.1 mol/L, with a low detection limitation of 0.06 mmol/L. It is confirmed that the fluorescence of PCN could be effectively decayed by the photoinduced charge transfer via the adsorbed Cl-for trapping holes, mainly by means of the time-resolved fluorescence and surface photovoltage spectra. The porous structure and modified Ag promote the adsorption of Cl-on resulting Ag-PCN, leading to excellent fluorescence detection for Cl-. This work provides a feasible route to develop a fluorescence detection of Cl-with g-C3N4 nanosheets in environment water.