$Eu^{2+}/Dy^{3+}$-doped $Sr_2MgSi_2O_7$ powders were synthesized using a solid-state reaction method with flux ($NH_4Cl$). The broad photoluminescence (PL) excitation spectra of $Sr_2MgSi_2O_7:Eu^{2+}$ were assigned to the $4f^7-4f^65d$ transition of the $Eu^{2+}$ ions, showing strong intensities in the range of 375 to 425 nm. A single emission band was observed at 470 nm, which was the result of two overlapping subbands at 468 and 507 nm owing to Eu(I) and Eu(II) sites. The strongest emission intensity of $Sr_2MgSi_2O_7:Eu^{2+}$ was obtained at the Eu concentration of 3 mol%. This concentration quenching mechanism was attributable to dipole-dipole interaction. The $Ba^{2+}$ substitution for $Sr^{2+}$ caused a blue-shift of the emission band; this behavior was discussed by considering the differences in ionic size and covalence between $Ba^{2+}$ and $Sr^{2+}$. The effects of the Eu/Dy ratios on the phosphorescence of $Sr_2MgSi_2O_7:Eu^{2+}/Dy^{3+}$ were investigated by measuring the decay time; the longest afterglow was obtained for $0.01Eu^{2+}/0.03Dy^{3+}$.