<p>At present, research on dry powder for magnesium fire extinguishing is mostly focused on extinguishing surface fires, with limited research on internal smoldering. Moreover, fire extinguishing performance tests of dry powder are primarily conducted on horizontal surfaces, while inclined and vertical combustion poses greater hazards and extinguishing challenges. To address this issue, a new dry powder extinguishing agent (NaCl/PMHS@LMG) with a dual phase-change temperature range was prepared in this paper to extinguish surface open flames and internal smoldering generated by magnesium fires on non-horizontal surfaces. The extinguishing agent was made using sodium chloride (NaCl) and polymethylhydrosiloxane (PMHS)-modified low melting point glass powder (LMG). The results of material testing showed that the NaCl/PMHS@LMG exhibited a dual phase transition, featuring a lower initial phase transition temperature, greater thermal stability, and enhanced heat absorption. At the same time, the modification of PMHS converted LMG from hydrophilic to hydrophobic, enhancing the dry powder’s overall moisture resistance. The results of magnesium fire extinguishing experiments show that NaCl/PMHS@LMG can effectively extinguish magnesium fires in both stacked and vertical forms. Compared with NaCl, the average extinguishing time and consumption of NaCl/PMHS@LMG in stacked fires were reduced by 49% and 52.7%, respectively, and the temperature reduction rate of stacked surfaces during extinguishing was approximately 4 times faster than with NaCl. Additionally, NaCl/PMHS@LMG formed a three-layer air-isolated cover on the stack surface after extinguishing, with an “auto-filling” function that effectively extinguished internal smoldering in magnesium chip stacks. Subsequently, the extinguishing mechanism of the dry powder was further explored.</p>