Mercuric ion (Hg2+) is a toxic metal ion in the environment, which will seriously damage the people's health. Therefore, the simple sensitive detection of Hg2+ is of great significance. In this work, Ag-CoFe2O4/reduced graphene oxide (rGO) nanocomposites were synthesized via a one-pot microwave-assisted reaction, which can directly oxidize 3, 3', 5, 5'-tetramethylbenzidine (TMB) to produce a light blue. Then we have developed a dual colorimetric and SERS detection of Hg2+ based on the stimulus of intrinsic oxidase-like catalytic activity of AgCoFe2O4/ rGO nanocomposites. It is demonstrated that the interaction between Hg2+ and Ag nanoparticles can occur in a short time, which includes the formation of Ag-Hg alloy due to the reduction of Hg2+. In addition, the formation of such alloy can enhance the oxide-like activity, which makes the detection of Hg2+ more sensitive. By using the SERS detection approach, the assay can detect Hg2+ as low as 0.67 nM. This detection ability is much better than previous reports based on the enzyme-like catalytic reaction, which is also lower than the maximum value of Hg2+ permitted in drinking water by the World Health Organization (WHO) (30 nM) and United States Environmental Protection Agency (EPA) (10 nM). In addition, this detection system also shows an excellent selectivity toward Hg2+ due to the affinity of Hg to Ag-CoFe2O4/rGO nanocomposites. Therefore, this approach is potentially applicable for the sensitive determination of Hg2+ in real environmental conditions.