Au@Pt NPs have an excellent peroxidase like activity，and silver ions(Ag+) could intensively inhibit the catalytic ability of Au@Pt NPs.Based on this，a colorimetric detection of Ag+ was performed in a linear range of 0.1- 10 nmol/L with a detection limit of 0.05 nmol/L.The sensing system also showed a sensitive response to Hg2+.A linear range of 10-200 nmol/L and a detection limit of 5.0 nmol/L were achieved in the colorimetric detection for Hg2+.The method was applied in detection of tap water spiked with Ag+(10，50,100 nmol/L)，and the recoveries ranged from 83.8% to 97.7% with relative standard deviations(RSD) of 3.0%-9.6%.This method has the advantages of simplicity，sensitivity and low cost.