As the inverse-opal structure facilitates the separation of electron–hole pairs and electron transfer, it may generate many radical species with strong oxidation capability. When a low bias voltage was applied on the TiO₂ electrodes with inverse-opal structure, they exhibited more excellent photoelectrochemical properties and photoelectrocatalytic activity than TiO₂ film under simulated solar light irradiation. When different types of active species scavengers were added, the different performances of TiO₂ photonic crystals in rhodamine B degradation showed that besides ˙OH and holes, which were the main active species in the photocatalysis, O₂˙⁻ played a vital role in the photoelectrocatalytic degradation process. Furthermore, the stronger signal of ˙OH-trapping photoluminescence and the variation in the concentration of nitroblue tetrazolium reflected that more ˙OH and O₂˙⁻ could be generated in the photoelectrocatalysis than that in the photocatalysis, and O₂˙⁻ was partially obtained from the cathode surface. At last, the roles active species played in the photoelectrocatalytic and photocatalytic processes were compared, and the possible degradation mechanisms of TiO₂ photonic crystals in photoelectrocatalytic and photocatalytic systems were put forward, which could provide a good insight into the mechanism of photoelectrocatalytic degradation on TiO₂ photonic crystals.