Zinc strontium phosphate glasses doped with different trivalent praseodymium ion (${\mathrm{Pr}}^{3+}$) concentrations are presented and their photoluminescence properties are investigated upon 442 nm excitation. With the ${\mathrm{Pr}}^{3+}$ concentration decreasing, the orange emission of ${\mathrm{Pr}}^{3+}$ (${}^1{\mathrm{D}}_2{}^3{\mathrm{H}}_J$) is enhanced steadily at the cost of its blue emission (${}^3{\mathrm{P}}_{\mathrm{1,0}}{}^3{\mathrm{H}}_4$). Monochromic orange emission of ${\mathrm{Pr}}^{3+}$ ions is obtained when the ${\mathrm{Pr}}^{3+}$ doping is reduced to 0.05 mol.%. The mechanism controlling the monochromatic characteristic of ${\mathrm{Pr}}^{3+}$ emissions is supposed to be associated with the phonon-aided nonradiative relaxation process of ${\mathrm{Pr}}^{3+}$: