In our previous study, Eu ions in KSrPO4 were reduced from trivalent to divalent by irradiation with a Nd:YVO4 nanosecond laser at a wavelength of 266 nm. This reduction was caused by laser light, so this phenomenon was termed photoreduction. Here, the effect of the laser fluence and charge compensation defects on photoreduction were investigated to model the process. The dependence of the amount of Eu ions reduced on the photoluminescence intensity suggests that a two-photon process is involved. Considering the energy of the laser wavelength at which photoreduction occurs, the two photons were attributed to the excitation of [Eu3+-O2-] charge transfer and charge transfer from a Sr vacancy to the conduction band, respectively. KSrPO4:Eu phosphors co-doped with various metal ions were synthesized, and photoreduction was conducted on these phosphors. In the case of co-doping trivalent metal ions, the amount of reduced Eu ions was increased, while the amount was decreased when co-doping monovalent ions. We consider that this difference is due to the change in the defect state of KSrPO4:Eu by co-doping metal ions and suggest that the presence of charge compensation defects, such as Sr vacancies, are important in the photoreduction process. Based on these results, we model the photoreduction process and discuss the details of the mechanism.