Ultrasensitive detection of heavy metal ions through electrochemical methods is a longstanding research topic. One of the most compelling subjects is preparation of modified electrodes with a high specific surface area as well as fast response characteristics. In this study, novel bismuth oxide surface-decorated nanoporous bismuth (BiO@NPBi) was prepared through a typical dealloying method, where a bicontinuous ligament–channel structure greatly improved the electrically active surface area and electron conduction. The BiO@NPBi-modified electrode was employed to detect heavy metal ions using square wave anodic stripping voltammetry (SWASV), and the corresponding limits of detection (LOD) were 0.02 and 0.03 µg/L for simultaneous detection of Pb and Cd. Due to the high cycling stability and capacity of bismuth oxides, the relative standard deviation (RSD) of the response currents using BiO@NPBi was less than 4.16%, which is very considerably lower than the RSD of NPBi. Furthermore, a BiO@NPBi-modified electrode was used to determine the concentration of Pb and Cd in tap water samples, and excellent recovery figures were obtained as there is little interference from other co-existing ions. In conclusion, the BiO@NPBi-modified electrode showed good reusability and feasibility for quality assurance in agricultural, food and environmental applications.