Heavy metal pollution has received great attentions in recent years. The traditional methods for heavy metal detection rely on the expensive laboratory instruments and need time-consuming preparation steps; therefore, it is urgent to develop quick and highly sensitive new technologies for heavy metal detection. The colorimetric method based on the gold nanoparticles (AuNPs) features with simple operation, high sensitivity and low cost, therefore, enabling it widely concerned and used in the environmental monitoring, food safety and chemical and biological sensing fields. This work developed a simple, rapid and highly sensitive strategy based on the Fenton reaction and unmodified AuNPs for the detection of Cu2+ in water samples. The hydroxyl radical (·OH) generated by the Fenton reaction between the Cu2+ and sodium ascorbate (SA) oxidized the single stranded DNA (ssDNA) attached on the AuNPs surface into variable sequence fragments. The cleavage of ssDNA induced the aggregation of AuNPs in a certain salt solution, therefore, resulting in the changes on the absorbance of solution. The assay conditions were optimized to be pH value of 7.9, 11 mg·L-1 ssDNA, 8 mmol·L-1 SA and 70 mmol·L-1 NaCl. Results showed that the absorbance ratio values at the wavelengths of 700 and 525 nm (A700/A525) were linearly correlated with the Cu2+ concentrations. The linear detection range was 0.1～10.0 μmol·L-1 with a detection limit of 24 nmol·L-1 (3σ). Spiked recoveries ranged from 87%～120% in three sorts of water, including drinking water, tap water and lake water, which confirmed that the potentials of the proposed assay for Cu2+ detection in reality.