Non-lead metal halide double perovskites (MHDPs), recognized as one of the most promising alternatives to lead-based metal halide perovskites (MHPs), have received enormous attention in recent years due to their nontoxicity and good thermodynamic stability. However, the development of a broadband near-infrared (NIR) emitting MHP with high optical efficiency and robust chemical stability remains a challenge. In this work, we report a broadband NIR emitting lead-free MHDP Cs2SnCl4Br2 activated by Sb3+ with the largest full width at half maximum of 164 nm. The morphology and particle size were controllably evolved via finely adjusting the preparation temperature. Most surprising is that the high-temperature post-treatment, which is to be avoided always, was applied for the as-obtained NIR MHDP, achieving an unexpectedly great boosting of the NIR emission efficiency by 13 times. Moreover, excellent stability was achieved, which showed that the broadband NIR emission intensity retains 90% of the initial level after continuous UV irradiation for 48 h, maintains 100% of the initial level after being immersed in water for 6 h, and increases up to 102% after being stored in air for 2 months. The origin of NIR emission from Sb3+ ionoluminescence was verified with the combination of experimental and DFT studies. As a proof-of-concept, a broadband NIR light emitting diode with a radiant flux of 17.23 mW was fabricated using the as-obtained broadband NIR MHDP for NIR spectroscopy applications. This work not only provides a method for the rational design of broadband NIR MHPs and extending their applications, but also prompts the steps to develop novel MHDPs with superior chemical and optical stability.