A colorimetric method was developed to determine chromium ions(Cr3+) in water,by using 13 nm diameter gold nanoparticles(AuNPs) synthesized by the citrate-mediated reduction of HAuCl4 as the transducer component for signaling color change,and 3-amino-5-mercapto-1,2,4-triazole(AMT) as the recognition element for Cr3+.AMT was assembled onto the surface of AuNPs to form a stable water-soluble AMT-AuNPs compound.In the presence of Cr3+,the specific binding of AMT and Cr3+ in solution could lead to the aggregation of AMT-AuNPs and a red shift of the maximum absorption wavelength,simultaneously producing an obvious color change from red to blue-purple,which could be used for Cr3+ visual detection.Under the optimized conditions(concentration of AMT:0.8 μmol/L；pH 7.0),the detection limit(3σ) of the method could be as low as 100 nmol/L for Cr3+ with linear detection range of 6-14 μmol/L,and there was almost no interference from other metal ions.With the advantages of fast response(5 min)，simplicity of preparation and manipulation,and no need of reading device,the presented method is promising in the application of on-site fast detection of Cr3+ in water.