In this study, a sensitive colorimetric method for the detection of copper ion (Cu2+) was developed based on controlling the peroxidase-like activity of (gold core)@(ultrathin platinum shell) nanocatalysts (Au@Pt-NCs). It was found that D-penicillamine can effectively inhibit the activity of Au@Pt-NCs. After being incubated with Cu2+, D-penicillamine lost inhibition toward the catalytic ability of Au@Pt-NCs. Based on the above interaction, a colorimetric detection of Cu2+ was develop by measuring the colorimetric signal variation of the H2O2-3,3',5,5'-tetramethylbenzidine (TMB) reaction. This method exhibited high sensitivity and selectivity toward Cu2+ over a panel of other metal ions. The detection limit of this method was 3.7 nM and the linear range was 20 - 300 nM. Moreover, 20 nM Cu2+ can be distinguished directly by the naked eye. Furthermore, this method was applied to the analysis of water samples with good accuracy. These results demonstrated the excellent application potential of the method.