Zinc strontium phosphate glasses doped with different trivalent praseodymium ion (Pr3+) concentrations are presented and their photoluminescence properties are investigated upon 442 nm excitation. With the Pr3+ concentration decreasing, the orange emission of Pr3+ (D21 HJ3) is enhanced steadily at the cost of its blue emission (P1,03 H43). Monochromic orange emission of Pr3+ ions is obtained when the Pr3+ doping is reduced to 0.05 mol.%. The mechanism controlling the monochromatic characteristic of Pr3+ emissions is supposed to be associated with the phonon-aided nonradiative relaxation process of Pr3+: Pj3→