The development of high-performance photoelectrochemical (PEC) sensors for chemical oxygen demand (COD) monitoring represents a critical advancement in water quality assessment technology. This study presents a novel TiO2 nanowire/WO3-x nanosheet (TiO2/WO3-x) composite photoanode fabricated through an in-situ hydrothermal approach, demonstrating exceptional PEC properties for COD measurement. Strategic introduction of interfacial oxygen vacancies at the heterojunction significantly enhances charge carrier separation and transport efficiency, resulting in a remarkable improvement in photocurrent response. The unique hierarchical architecture, combining TiO2 nanowires with WO3-x nanosheets, not only provides an abundance of catalytic active sites but also enhances the efficient oxidation of organic pollutants. The optimized sensor exhibits superior analytical performance, including a wide linear measurement range (1.125–480 mg/L) and an impressive detection limit of 0.72 mg/L. Comprehensive evaluation reveals excellent anti-interference capability against common inorganic ions (Cl-, NO3-, K+) and remarkable operational stability over extended periods (>30 days). Practical application in real water sample analysis demonstrates high reproducibility (RSD < 8 %) and accuracy (relative error: 2.72–7.12 %). This work introduces a new paradigm for defect engineering in PEC sensors, providing a robust solution for real-time, in-situ COD monitoring in various aquatic environments.