An intuitively persistent enhancement of the local surface potential of BiFeO3 layers in both heterostructures of BiFeO3/SrRuO3/SrTiO3 and BiFeO3/Sr0.09Nb0.01TiO3 was observed by the Kelvin probe force microscopy technique after the illumination of 375 nm laser. This photo-induced enhanced surface potential can maintain as long as 15 h after the illumination. We attributed this super-long-time relaxation of photo-induced influence to a photo-induced depolarization in the BiFeO3 thin films, and our first-principles calculation of double-potential well further provides an instinct understanding on this depolarization process. Our findings provide a peculiar understanding into the photo-induced phenomena on the widely researched ferroelectric systems and offer an approach to tune their multifunctionality of the magnetization and polarization not only by applied magnetic and electric fields but also by optical filed.