Yttrium aluminum garnet (YAG): Pr3+ is synthesized by codeposition me thod. Vacuum ultraviolet (VUV) excitation and emission spectra of YAG: Pr3+ at different temperatures are measured using synchrotron radiation as an excitation source. We analyze the transition process after excitation to 4f5d en ergy band and estimate the Huang-Rhys factor S and phonon energy ηω of YAG: Pr 3+ by multi-phonon relaxation. Emission of self-trapped exciton (STE) and 3P0→3H4,5,6 emission transferr ed from the STE energy appear when YAG: Pr3+is excited by 170nm, whic h is a different transition process from those excited by 240nm and 289nm.