A series of TiO2 nanorod array electrodes with different lengths have been successfully fabricated by a controlled hydrothermal method for photoelectrochemical (PEC) application. In order to enhance the conductivity of the TiO2 nanorods and enable the PEC activity under visible light irradiation, the TiO2 nanorod samples have been further hydrogenated. The influence of the length of the hydrogenated TiO2 nanorod arrays (H-TNRs) on their visible-light-driven photoelectrocatalytic activity was investigated. With increasing the length of the H-TNRs to about 3.0 μm, the activity was close to the maximum. Subsequently, the H-TNRs photoanode was fitted into a thin-layer photoelectrochemical cell for mineralization of organic compounds. The visible light PEC performance was enhanced so much that chemical oxygen demand (COD) detection was achieved under visible light as the light source for the first time. The excellent relationship between the photoelectrochemical COD and conventional COD values within the range of 0–288 mg L−1 suggests that the simple visible light driven PEC method is a promising alternative to the conventional COD method.