Near-infrared (NIR) light-emitting materials are crucial for night vision, in vivo-imaging, and secret codes. However, issues such as thermal quenching and chemical stability tend to significantly suppress the efficiency. In this work, we modify NIR-emitting phosphor Y(Al,Mg)(Al,Si)O:Cr (YMAS:Cr) by substitution via 2[Al] → [Mg] + [Si] based on prototype YAlO:Cr. The YMAS:Cr phosphor shows extraordinary optical performance and chemical stability, with 86–93% quantum yield (QY), 94% room temperature emission intensity at 150 °C, and almost no luminescent loss after 8 days in water and heating at 1000 °C. The substitution lifts the local symmetry, leading to a weak crystal field, spin-allowed T→A transition of Cr in YMAS:Cr, and the higher efficiency and broadening emission spectra compared to the prototype YAlO:Cr (QY = 16%) of spin-forbidden E→A in the strong crystal field. Potential applications are demonstrated by using YMAS:Cr in a high-power output light-emitting diode and luminescent ink. This work proposes the strategy via altering the local effect for improving luminescence properties, potentially stimulating further research on NIR-emitting materials.