This paper reports a low-cost and highly sensitive flexible sensor for the determination of lead and cadmium in tap and lake water. This sensor achieved low detection limits and high performance by decorating cation-exchanging resin and in situ deposited bismuth on the working electrode. In situ deposited bismuth film was much easier to fabricate and formed a highly porous 3-dimensional structure, which provided more surface area for the reaction of the target ions. The cation-exchanging Nafion film can help reduce the interference caused by surface active compounds and enhance the pre-concentration of lead and cadmium at the same time. Flexible polypropylene film was chosen as the substrate to reduce the cost and expand the sensor’s applications. Finally, the detection limits were 1.68 ppb for Pb and 1.24 ppb for Cd at the deposition time of 300 s. Particularly, this sensor revealed good linearity (R2 = 0.993) at a wide range of 10–500 ppb for cadmium in tap water environment. The proposed sensor was successfully applied to lead and cadmium determination in environmental water samples and the results were in good agreement with those from a standard procedure.