A simple and cost-effective colorimetric approach based on unmodified Au@Ag nanorods (Au@Ag NRs) was developed for Hg²⁺ detection. Unmodified Au@Ag NRs with different Ag nanoshell thicknesses served for the signal readout because the Ag coating-induced a blueshift and enhancement of the longitudinal plasmon of Au NRs, resulting in abundant and tunable optical absorptions in the visible region. The etching sensing mechanism was revealed to be related to the redox reaction between Hg²⁺ and the Ag nanoshell of the Au@Ag NRs. The Ag nanoshell of the Au@Ag NRs was gradually etched from the ends as the Hg²⁺ concentration was gradually increased, and shoulder shapes were formed, and then disappeared. The Hg²⁺ concentration-dependent color of Au@Ag NRs with a thick Ag nanoshell thickness changed from brownish-red to light-red, light-violet, and finally to colorless. The limit of detection (LOD) and the detection range of Hg²⁺ became tunable as the Ag nanoshell thickness increased, and the lowest LOD was 10 nM. A dip located between two strong absorption peaks was observed when Au@Ag NRs with a thick Ag nanoshell thickness were used, and the change in this dip provided a new sensor parameter for Hg²⁺ detection on the basis of absorption spectra. The proposed method also showed a high selectivity toward Hg²⁺ over other metal ions. The Au@Ag NR detection system could detect even low Hg²⁺ concentrations in drinking water.