Sr9Mg1.5(PO4)(7):Eu2+ has recently been reported as a promising blue light-excited orange-yellow phosphor that can be used in white LED device. Here, Ce3+-codoping is found to be an effective strategy to improve the luminescence performance of Sr9Mg1.5(PO4)(7):Eu2+ phosphor. The coexistence of Eu2+ and Eu3+ ions has been verified via photoluminescence spectral analysis. The reduction of Eu3+ to Eu2+ in Sr9Mg1.5(PO4)(7) lattice cannot be completed in a reducing atmosphere, but can be promoted through codoping with Ce3+ ions to a great extent, which finally increase the effective concentration of Eu2+ in the crystal lattice. The Eu3+-Eu2+ reduction mechanism is analyzed using a charge compensation model. This work not only achieves enhanced luminescence of the Sr9Mg1.5(PO4)(7):Eu2+ phosphor by codoping with Ce3+ ions, but also provides new insights into the design of Ce3+/Eu2+ codoped luminescent materials.