Abstract: Flexible fiber-based electrochemical sensors hold promising application prospects in the field of wearable health monitoring. Polyester fiber boasts comprehensive outstanding properties and serves as an ideal substrate for conductive fibers. Nevertheless, its chemically inert surface tends to weaken the interfacial adhesion of conductive coatings and impair mechanical service stability, which hinders the practical implementation of corresponding devices. In this work, a polydopamine interfacial engineering strategy is adopted. Polydopamine provides abundant active sites to induce the uniform growth of poly(3,4-ethylenedioxythiophene), thereby fabricating conductive polyester fibers coated with compact and stable conductive layers. Sodium ion and potassium ion fiber sensing electrodes are constructed by loading carbon nanotubes, selective membranes and reference layers onto the as-prepared conductive polyester fibers. The mechanical properties and sensing performance of the as-prepared fibers were measured and compared with a micro-tensile tester and electrochemical workstation. The resultant fiber sensing electrodes integrate favorable flexibility, robust mechanical durability and superior electrochemical performance, featuring fast response (0.1 s), high selectivity, and broad linear detection ranges (Na⁺: 10⁻⁴—0.5 mol/L; K⁺: 0.1—100 mmol/L). Such electrodes can be stably integrated into textiles, diapers and other carriers, offering a reliable fabrication strategy and material platform for the practical application of fiber sensors toward wearable body fluid health monitoring.