Abstract: To effectively control the explosion intensity of hydrogen–air mixtures in confined spaces and elucidate the suppression mechanism of micron-sized water mist containing dimethyl methylphosphonate (DMMP, O=P(CH3)(OCH3)2), this study combines constant-volume combustion bomb experiments with chemical kinetic simulations using Chemkin-Pro. Results indicate that water mist containing O=P(CH₃)(OCH₃)₂ promotes the formation of cellular structures on the flame front, thereby inducing flame instability. At equivalence ratios (Φ) of 0.8, 1.0, and 1.5, the O=P(CH3)(OCH3)2-laden water mist effectively reduces the average flame speed (with reductions ranging from 24.2% to 47.2%) and suppresses the formation of tulip flames, which are replaced by wrinkled flame structures. The mist suppresses the pressure rise rate by reducing the laminar flame speed, but simultaneously enhances flame instability, which tends to increase the pressure rise rate. The overall suppression performance (with pressure reduction ranging from 41.0% to 65.8%) results from the coupling of these two opposing effects. Additionally, the O=P(CH3)(OCH3)2-laden mist achieves effective explosion suppression by reducing the concentrations of H∙, O∙, and OH∙ radicals, with reductions exceeding 80%. The physical suppression arises from pre-flame cooling and dilution effects of the water mist, while the chemical suppression is attributed to the decomposition of O=P(CH3)(OCH3)2 into phosphorus-containing radicals such as HOPO∙, HOPO2∙, HPO2∙, PO(OH)2∙, and PO(H)(OH)∙. These species scavenge reactive H∙ and OH∙ radicals, promoting the formation of stable products like H2 and H2O, thereby interrupting the chain reactions in hydrogen-air explosions.