Defect engineering has been demonstrated to be an appealing strategy to boost the photocatalytic activity of materials. However, can higher defect concentration bring about higher photocatalytic activity? This is an open question. In this work, BiPO4photocatalysts with controllable oxygen vacancy concentrations were successfully synthesized. The photocatalytic activity of the obtained BiPO4photocatalysts was determined by the removal of ciprofloxacin and 4-chlorophenol, as well as CO2photoreduction. The BiPO4materials with lower oxygen vacancy concentration could display unexpected higher photocatalytic efficiency. Through the investigation of different factors which may affect the photocatalytic performance, such as crystal structure, morphology, specific surface area, defect, and energy band structure, it can be found that the energy band structure difference was responsible for the enhanced photocatalytic activity.