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3D bare TiO2 nanotube arrays loaded with Au nanoparticles (3D Au-NPs/TNTAs) were prepared to effectively enhance the photoelectrocatalytic properties. A simple flow injection sensing system was proposed for the real-time on-line determination of chemical oxygen demand using the 3D Au-NPs/TNTAs electrode. The proposed photoelectrochemical oxidative degradation principle of the sensing system was analyzed and validated by the representative organic compounds with known COD values at a potential bias 1.5 V under UV illumination. A practical detection limit of the sensing system is 0.18 mg/L and the linear range is 1.92–3,360 mg/L under the optimum conditions. In the experimental process, the sensing system exhibits long-term stability and good reproducibility.