The modification of ternary metal oxide to improve the photoelectrocatalysis (PEC) properties of TiO2 photocatalyst is a hot issue in environmental and resource applications. Herein, we present a novel photoelectrocatalytst of Se doped-TiO2/Ti nanotube arrays (Se@TiO2/Ti-NTAs) for high-efficiency degradation of diazinon pesticide. The Se@TiO2/Ti-NTAs were prepared to treat the TiO2-NTAs in Se-TiO2 sol-gel for 10 min, which was followed by calcination in air at 200 °C for 1 h. Optical absorption spectroscopy of the Se@TiO2/Ti-NTAs indicated that there is a red-shift in the optical energy gap to 2.95 eV if compared to the pristine TiO2/Ti NTAs, which suggests that the new photoelectrocatalyst is photoactive under visible light irradiation. We evaluated the photoactivity of the sample by using it as the photoelectrocatalyst in the degradation of diazinon pesticides under the PEC process. The results showed that the Se@TiO2/Ti-NTAs can degrade 95.62% for 1 h under visible light irradiation, which is equivalent to the degradation rate constant of 0.0183 s−1. For comparison, the pristine TiO2/Ti NTAs only degrades the diazinon as high as 87.65%, even then under UV light irradiation. Our result also indicated that the Se@TiO2/Ti-NTAs promote active photoelectron transfer and active radical formation, such as •OH and •O-2, for rapid diazinon pesticide degradation. The Se@TiO2/Ti-NTAs photoelectrode should be a potential platform for environmental pollution treatment.