One of the hot spots of nanomaterials research is the preparation of carbon-dots ( C-dots) by simple steps with cheap raw materials and looking for its potential application. In this study, coal-based C-dots was prepared from coal mined of Wucaiwan in Xinjiang by mixed acids ( H2 SO4+HNO3 )/ultrasound treatment, and at the same time of structural characterization, the coal-based C-dots were used as fluorescent probe to detect metal ions in water. It was found that the coal-based C-dots were polycyclic aromatic hydrocarbons with particle size of (8±4) nm linked with oxygen-containing groups such as nitro group, possessing the annulus wall of multilayer graphene fragment structures built up by sp2 carbons. This endowed the coal-based C-dots with good dispersity in water, high absorbance and strong fluorescence. The coal-based C-dots were used as viable probes in that their fluorescence was selectively quenching by CuⅡ. The finding was exploited to design a fluorometric assay for CuⅡ with a detection limit of 9. 6 nmol/L.