Near-infrared (NIR) phosphor-converted light emitting diode (pc-LED) light sources have broad application prospects in environmental science, biomedical and plant growth fields. However, NIR phosphors still suffer from narrowband emission and low thermal stability. Here, we prepared a novel Lu2CaMg2-xSi3O12:xCr3+ (LCMS:xCr3+, x = 0.005-0.06) phosphor with broadband emission by a high-temperature solid state reaction. Under excitation at 450 nm, the emission spectrum of the LCMS:0.05Cr3+ phosphor shows a broadband emission in the range of 650-1000 nm with a large full width at half maximum (FWHM) of 125 nm and an R-line emission of 692 nm. In addition, the LCMS:0.05Cr3+ phosphor has good thermal stability and can maintain 70% emission intensity at 150 °C relative to that at room temperature. The LCMS:0.05Cr3+ phosphor exhibits a high internal quantum efficiency (IQE) of ∼76%. Using this phosphor, a NIR pc-LED with photoelectric efficiencies of 14.8% at 100 mA and 6.0% at 320 mA and NIR output powers of 59.5 mW at 100 mA and 181.0 mW at 320 mA was obtained. The broadband LCMS:0.05Cr3+ phosphor with a NIR emission peaked at 750 nm can serve as a light source for plant cultivation and has superior application prospects in the agriculture field.