A series of Sr_1.98(Al_1–xMg_x)(Al_1–xSi_1+x)O₇: 2%Eu²⁺ phosphors is prepared by the high-temperature solid-state reaction method, the crystal structures and luminescent properties of the prepared phosphors are investigated by measuring the X-ray diffraction, luminescent spectra and optical microscope. The isomorphic compounds of Sr₂Al₂SiO₇ and Sr₂MgSi₂O₇ contain tetrahedra including MgO₄, SiO₄ and AlO₄. Although the valences of the MgO₄^6–, SiO₄^4– and AlO₄^5– groups are different, the charge imbalance occurs when the MgO₄^6– and SiO₄^4– substitutes of AlO₄^5– and AlO₄^5–, respectively. While the groups are co-substituted, the charge imbalance disappears. And the larger volume of MgO₄ and the smaller volume of SiO₄ together replaces the similar volume of AlO₄, resulting in the decrease of (Si/Al)O₄ and increase of (Mg/Al)O₄. Moreover, the decrease of unit cell parameters c and the increase of a and V due to the increased replacement of Mg²⁺ (0.57 Å for CN = 4) by Al³⁺(0.39 Å for CN = 4) and Si⁴⁺ (0.26 Å for CN = 4) by Al³⁺ (0.39 Å for CN = 4) cause the ambient temperature to change, the crystal field splitting of the Eu²⁺ cation to be weakened, and the emission spectra to be blue-shifted from 503 nm to 467 nm, which are closely related to the local coordination environment of the Eu²⁺, in addition, this reveals that the emission color of this series of phosphors can be tuned from green with color coordinate (0.2384, 0.3919) to blue (0.1342, 0.1673) by adjusting the chemical compositions via the MgO₄^6– and SiO₄^4– groups’ co-substitution for AlO₄^5–. The full width at half maximumof emission band is 120 nm when x = 0, the photoluminescence emission width decreases monotonically from 89 to 50 nm as x is increased from 0.25 to 1. In other words, the full width at half maximum of emission band exhibits a decreasing trend. The internal quantum efficiency is enhanced with increasing x in Sr_1.98(Al_1–xMg_x)(Al_1–xSi_1+x)O₇: 2%Eu²⁺ phosphors. These results verify that the groups’ substitutions are enhanced with polyhedron changing in the solid solutions and contribute largely to the luminescence properties of the phosphor.