The rapid advancement of near-infrared light source technology has positioned phosphor-converted lightemitting diodes (PC-LEDs) operating within the near-infrared spectrum at the forefront of interdisciplinary research, with extensive applications in biomedicine, food quality assessment, and agricultural monitoring. However, the development of light sources that cover the second near-infrared spectral window (NIR-II, 1000-1700 nm) continues to encounter significant technical challenges. Inspired by chemical unit cosubstitution strategy, we successfully developed a broadband NIR-II phosphor designated as (Li0.7Mg0.3) (Ga0.7Mg0.3)TiO4:0.003Ni2+ ((L0.7M0.3)(G0.7M0.3)T:0.003Ni2+) through [Mg2+-Mg2+] dual-site substitution in orthorhombic LiGaTiO4 (LGT) normal spinel. This co-substitution strategy resulted in an impressive enhancement of the internal quantum efficiency by 29.77 %-64.8 %, while maintaining thermal stability without significant attenuation at elevated temperature. To validate its practical utility, we fabricated prototype for a NIR phosphor-converted LED by integrating (L0.7M0.3)(G0.7M0.3)T:0.003Ni2+ a commercial 340 nm chip. The resulting device demonstrates promising potential for applications in food safety inspection and deep-tissue infrared imaging. This work provides a perspective for designing broadband high-efficiency NIR-II phosphors via modulation of crystal field environment.