The silver ion（Ag⁺） poses serious threats on the environment and the health for human. Therefore， the accurate and interference-free detection of Ag⁺ is of great scientific significance with practical value of application. The surface-enhanced Raman spectroscopy（SERS） sensing detection technology based on functionalized gold nanoparticles was established to realize the accurate and interference free detection of Ag⁺. When Ag⁺ was present， adjacent gold nanoparticles were connected and underwent controlled aggregation to form a chain-like structure based on the coordination effect between Ag⁺ and cyanogroup（-CN） in merhydryl benzonitrile（MBN） modified on the surface of gold nanoparticles， generating a “hotspot” effect. The SERS signal of MBN at 2 228 cm^-1 in the biological “silent zone” was enhanced， and the degree of enhancement increased with the increase of the concentration of Ag⁺. The results showed that the SERS sensing technology established for detecting Ag⁺ had a good linear relationship in the concentration interval of 0.1 nmol/L and 100 nmol/L. The minimum detectable concentration of Ag⁺ was 0.1 nmol/L， much lower than the limit requirement of the US Environmental Protection Agency that the concentration of Ag⁺ in drinking water should be less than 0.93 μmol/L. The detection with Ag⁺ spiked recovery was performed in actual water samples， with a recovery rate ranged from 99.7% to 101.4%.