Silver ion (Ag+) poses serious threat to the environment and human health, therefore, accurate and interference-free detection of Ag+ is of great scientific significance and practical application value. To achieve this purpose, the surface-enhanced Raman spectroscopy (SERS) sensing detection technology based on functionalized gold nanoparticles was established, when Ag+ were present, based on the coordination of Ag+ and cyanogroup (-CN) in merhydryl benzonitrile (MBN) modified on the surface of gold nanoparticles, the gold nanoparticles were controlled to gather, forming a chain structure and generating the "hot spot" effect, the SERS signal of MBN was enhanced at 2228 cm-1 in the biological "silent zone", and the degree of enhancement increased with the increase of Ag+ concentration. The results show that, in the concentration interval of 0.1 nM ~ 100 nM, the linear relationship is good, and the minimum detectable concentration of Ag+ is 0.1 nM, which is far lower than the Environmental Protection Agency (EPA) regulation that the concentration of Ag+ in drinking water which should be less than 0.93 μM. While the recovery rate of Ag+ was 99.7% ~ 101.4% in actual water samples.