Abstract: To enable real-time trace detection of cadmium ions (Cd²⁺) in aqueous solutions, this study proposes a micro/nano optical fiber sensor based on a polymer nanocoating of N-isopropylacrylamide-allylthiourea (PNA). The sensor employs a single-mode tapered four-core single-mode (STFS) structure, on which a —S—Cd²⁺—S— crosslinked structure are costructed between the PNA nanocoating and with Cd²⁺ in the test solution, leading to refractive index changes in the thin film on the micro/nano fiber surface. By monitoring the shift of interference dips in the transmission spectrum, highly sensitive detection of Cd²⁺ is achieved. Excellent linear response within a Cd²⁺ concentration range of 100~1000 nmol/L is achieved, with a detection limit as low as 2.20 μg/L, significantly below the water quality discharge standard stipulated by the World Health Organization (WHO). Selectivity experiments further confirm that the sensor’s response signal to Cd²⁺ is markedly higher than those to interfering ions such as Cr³⁺ and Ni²⁺, demonstrating superior specific recognition capability. This study provides a novel technical solution for rapid and highly sensitive detection of Cd²⁺ in water bodies, holding promise for applications in environmental monitoring and water quality safety.