Four series of isostructural solid solution phosphors with the composition of Sr8.95−xLnxMg1.5(PO4)7−x(SiO4)x:0.05Eu2+ (Ln = Y, La, Gd, Lu; x = 0−2) are synthesized. The influence of [Ln3+−Si4+] (Ln = Y, La, Gd, Lu) cosubstitution on the structure and luminescence properties of Sr9Mg1.5(PO4)7:Eu2+ phosphor is evaluated. Spectra analysis indicates that the valence state of Eu in Sr9Mg1.5(PO4)7:Eu2+ phosphor can be controlled from Eu3+ to Eu2+ by cosubstituting [Ln3+−Si4+] for [Sr2+−P5+], leading to an obvious enhancement of Eu2+ emission. The mechanism of abnormal reduction of Eu3+ ions in the lattice is discussed. In addition, a blueshift of Eu2+ emission is observed with increasing the [Ln3+−Si4+] content. These findings not only suggest that the as‐prepared Sr9−xLnxMg1.5(PO4)7−x(SiO4)x:Eu2+ phosphors are potential candidates for near‐UV‐based white light‐emitting diodes (LEDs) but also offer a novel way for Eu valence control and spectral tuning in Eu‐doped phosphors with multiple activator sites.