The optimal layout of the internal structure of a photoelectrocatalytic (PEC) reactor is essential for the degradation of pollutants in water. In this work, blue TiO₂ nanotube arrays electrode plates were prepared by an oxidation-reduction method. The influences of various layout factors of a PEC reactor on methylene blue (MB) degradation were investigated. Experimental results indicate that the MB degradation rate increased with the decreasing distance between the anode plate and the center of the light source, the decreasing rotation angle of the anode plate, and the increasing anode plate number. Further analysis results indicate that the influence of each layout factor on the MB degradation could all be ascribed to the ultraviolet (UV) fluence rate. The MB degradation rate shows a positively linear correlation with the UV fluence rate. The slope (0.00219) and intercept (0.0049) of the linear regression equation represent the relative efficiencies of UV photons used by anode catalytic oxidation and direct photolysis on MB degradation, respectively, and clarified the contribution of each reaction to the MB degradation rate. Moreover, comparison results of the PEC reactors with different inner diameters (64, 72 and 80 mm) show that a smaller inner diameter led to a faster MB degradation rate, but the PEC reactor with an inner diameter of 80 mm had the lowest consumption of electrical energy per order (E_EO) for MB degradation (17.24 kWh•m^-3•order^-1). Finally, the contributions of anode catalytic oxidation and direct photolysis to the MB degradation rate in PEC reactors with various inner diameters were quantitatively analyzed, which provides important reference for the optimal design of a PEC reactor.