In this study, an environmentally friendly and simple method to synthesize rGO–BiNP nanocomposite was newly developed to address some drawbacks of in-situ Bi plating method for heavy metal ions detection and the synthesized nanocomposite was confirmed using physical and electrochemical analysis. Furthermore, the feasibility of Cd ions detection by the synthesized nanocomposite was confirmed using square-wave anodic stripping voltammetry (SWASV) without in-situ Bi plating, which can change the pH value of test solution and the sensing results. Therefore, the fabricated sensor can directly test target solution without pre-treatment. Under the optimal conditions, the sensitivity for Cd ions was 49 nA/ppb and the correlation coefficient was 0.983, respectively. The limit of detection was 0.3 ppb for Cd ions for a short deposition time of 150 s. The fabricated sensor exhibited linear response to Cd ions in the concentration range from 10 ppb to 120 ppb with good linearity. The determination efficiency toward Cd ions was significantly enhanced by the synthesized rGO–BiNP coated working electrode, due to the enlargement of active area and its electrochemical properties. These results indicate a great potential of the rGO–BiNP electrode for electrochemical heavy-metal ion sensor applications.