The convenient chemical method for sensing mercury ion (Hg 2+ ) has drawn significant attention in recent researches. Herein we report a simple, ultrasensitive and inexpensive protocol to detect trace mercury ions in water without complicatedly tagging, based on the facile synthesis of a nanocomposition, which is constructed with inositol hexaphosphate (IP 6 ) stabilized gold nanoparticles (Au NPs) modified by crystal violet (CV) as Raman signal molecule and tri-sodium citrate (TC) as reducer, shortly named CV/TC/IP 6 /Au NPs. A possible mechanism for the Raman probe is the decreasing intensity of SERS band at 1173cm −1 of crystal violet (CV) with Hg 2+ addition due to the reduction of Hg 2+ by TC to Hg atom whose competition with CV molecules at the IP 6 /Au NPs surface, resulting in CV molecules detachment from the surface. The proposal Raman probe shows an unprecedented limit of detection (LOD) of 0.5pM for Hg 2+ . This SERS-based method of Hg 2+ determination could be expected to have wide-range applications in many areas such as analysis of trace Hg 2+ in river water.