Photochemical properties of the ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and its binary mixed solutions with organic solvent (DMF and MeCN) were investigated by laser photolysis at an excitation wavelength of 355 nm, using anthraquinone (AQ) as a probe molecule. It was indicated that the triplet excited state of AQ ((3)AQ*) can abstract hydrogen from [bmim][PF6]. Moreover, along with the change of the ratio of RTIL and organic solvent, the reaction rate constant changes regularly. Critical points were observed at volume fraction V-RTIL=0.2 for RTIL/MeCN and V-RTIL=0.05 for RTIL/DMF. For both systems, before the critical point, the rate constant increases rapidly with increasing V-RTIL; however, it decreases obviously with V-RTIL after the critical point. We conclude that the concentration dependence is dominant at lower V-RTIL, while the viscosity and phase transformation are dominant at higher V-RTIL for the effect of ionic liquid on the decay of rate constant.