In this paper, graphene and MnO2 nanoparticle were programmed to synthesize a nanocomposite, which was used to modified glassy carbon electrode (GCE). The presence of Gr-MnO2 nanocomposite has enhanced the electrochemical performance of the GCE towards the mercuric chloride (HgCl2). Electrochemical behaviors of HgCl2 on different electrodes were evaluated by cyclic voltammetry, the result shows that the Gr-MnO2 nanocomposite modified electrode showed higher chemical activity with HgCl2 than bare GCE, MnO2/GCE and Gr/GCE. In PBS solution (pH=5.5), the oxidation peak current raised as the concentration of HgCl2 increased from 12.1 to 210.8μM, the detection limit was 2.0μM (S/N=3). The modified electrode showed good selectivity, high stability and excellent regeneration.