Highly thermal stable Si–N-doped BAM (BaMgAl10O17: Eu2+) phosphors have been successfully synthesized by a mechanochemically assisted solid-state reaction method. Mechanical milling greatly improved the amount of Si–N pairs substituted for Al–O pairs in BAM lattice>. Si–N incorporation improves the photoluminescence (PL) properties and the color purity, reduces the thermal quenching, and most importantly, increases the thermal stability of the BAM phosphors significantly. The interpretation of the positive influence of Si–N doping was attributed to the local structure of the produced phosphors, which was analyzed with the aid of first-principles density functional calculations. This analysis showed that the substitution of Al–O pairs with Si–N pairs should preferentially occur in the boundary between the spinel layer and the conduction layer of the BAM phosphor, leading to a compression of the conduction layer. Eu2+ ions prefer to substitute the N-coordinated Ba2+ ions in the lattice of Si–N-doped BAM phosphors, leading to a strong Eu–N bonding. The results of these calculations agree fairly well with the results recorded experimentally, specifically the electron paramagnetic resonance (EPR) spectra, the X-ray absorption fine structure (XAFS), thermoluminescence spectra (TL), and decay behaviors.