In order to prevent and control the deflagration hazard of magnesium powder in fuel-lean conditions, the explosion suppression experiment device was used to test the effect of solid inerting agents (Mg(OH)₂, Ca(OH)₂, Ca(HCO₃)₂) on the explosion characteristics of magnesium powder. The particle size and concentration were considered. The results show that within the particle size range from 17 to 74 μm, the maximum explosion pressure of magnesium powder decreases with increasing particle size, and rises first and then falls as powder concentration increases. For the 17.0 μm magnesium powder, the optimal explosion concentration and the maximum explosion pressure are 350 g/m³ and 0.716 MPa, respectively. The addition of three inerting agents, namely Mg(OH)₂, Ca(OH)₂, and Ca(HCO₃)₂, both reduces the maximum explosion pressure and the maximum pressure rise rate of magnesium powder. The inerting ratios for effective and complete inerting of magnesium powder by the three agents were obtained. Among them, Mg(OH)₂ exhibits the best inerting performance, with the inerting ratios of 170% and 220% for effective and complete inerting, respectively. The inerting mechanism of solid inerting agents on magnesium powder under fuel-lean conditions is revealed. Mg(OH)₂ decomposes upon heating to produce MgO, which adsorbs onto the surface of magnesium particles and prevents the contact between magnesium and oxygen, thereby achieving inerting; Ca(OH)₂ exerts an inerting effect merely through thermal decomposition; Ca(HCO₃)₂ generates CO₂ by thermal decomposition, which further enhances the inerting performance. The obtained conclusions provide an important reference for realizing the effective inerting of magnesium powder explosion under fuel-lean conditions.