Nitrogen and sulfur co-doped green fluorescent carbon dots (G-CDs) with quantum yield (QY) of 28.26% were synthesized by hydrothermal method with p-phenylenediamine and thiosemicarbazone as precursors. The synthesized G-CDs exhibit good sensitivity and anti-interference ability for detecting copper ion (Cu2+) by fluorescence quenching (λex=390 nm, λem=535 nm) with a linear range of 20 to 200 μM and the limit of detection of 4.8 μM. This analytical method has been applied to detect Cu2+ of the local lake with good accuracy and recovery.