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Heavy metal pollution can be toxic to humans and wildlife, thus it is of great significance to develop rapid and sensitive methods to detect heavy metal ions. Here, a novel type of electrochemical sensor for the simultaneous detection of heavy metal ions has been prepared by using poly(L-glutamic acid) (PGA) and graphene oxide (GO) composite materials to modify the glassy carbon electrode (GCE). Due to the good binding properties of poly(L-glutamic acid) (PGA) for the heavy metal ions (such as Cu2+, Cd2+, and Hg2+) as well as good electron conductivity of graphene oxide (GO), the heavy metal ions, Cu2+, Cd2+, and Hg2+ in aqueous solution can be accurately detected by using differential pulse anodic stripping voltammetry method (DPASV). Under the optimized experiment conditions, the modified GCE shows excellent electrochemical performance toward Cu2+, Cd2+, and Hg2+, and the linear range of PG/GCE for Cu2+, Cd2+, and Hg2+ is 0.25–5.5 μM, and the limits of detection (LODs, S/N ≥ 3) Cu2+, Cd2+, and Hg2+ are estimated to be 0.024 μM, 0.015 μM and 0.032 μM, respectively. Moreover, the modified GCE is successfully applied to the determination of Cu2+, Cd2+, and Hg2+ in real samples. All obtained results show that the modified electrode not only has the advantages of simple preparation, high sensitivity, and good stability, but also can be applied in the field of heavy metal ion detection.