The presence of diclofenac (DCF) in water has developed into a severe environmental problem due to its pernicious effects on animals and human being. Herein, a photoelectrocatalytic (PEC) method using I doped TiO2 (I-TiO2) photoelectrode has been developed, which exhibited excellent visible activity in the degradation of DCF as well as pharmaceutical wastewater. The Langmuir Hinshelwood kinetic model, active species trapping experiments and analysis of intermediate products were employed to analyze the PEC degradation mechanism of DCF. The results revealed that the decomposition of DCF was primarily performed on the photoelectrode (I-TiO2) surface, and the degradation process was accompanied by the dechlorination reaction. The degradation of DCF was caused mainly by holes (66.6%) and hydroxyl radicals (27.6%) when Na2SO4 acted as electrolyte. The degradation rate of DCF could be significantly enhanced under the condition of high Cl(-)concentration (NaCl acted as electrolyte), which was caused by the participation of chloride radicals (Cl-center dot) in the PEC process. The Cl-center dot was the major factor contributing to the decomposition of DCF, and the holes and hydroxyl radicals played a less important role than Cl-center dot under this condition.