To investigate the influence mechanism of particle size of Al-Li-Mg alloy on its ignition and combustion behavior， Al-Li-Mg alloys with median particle sizes of 9， 13， 16， and 24 μm was selected and tested by using of the laser particle size analyzer， scanning electron microscopy， X-ray diffractometry， simultaneous thermal analyzer， oxygen bomb calorimeter， and a laser ignition experimental setup combined with high-speed photography and fiber optic spectrometry. Results show that with the increase in particle size of the Al-Li-Mg alloy， the ignition delay time sharply decreases and then stabilizes， dropping from 135 ms to 51 ms， and further to 15 ms and 18 ms. The combustion intensity decreases from 7300.4 to 1721.6， while the combustion duration slightly prolongs and then remains largely stable from 857 ms to 928 ms，until to approximately 920 ms. A comprehensive comparison indicates that the Al-Li-Mg alloy with a particle size of 13 μm achieves a balance among ignition delay （51 ms）， combustion duration （928 ms）， and combustion intensity （6041.8）. The study reveals that particle size affects the reaction pathway of the Al-Li-Mg alloy by regulating the competitive mechanism between heat conduction efficiency and elemental diffusion. An increase in particle size limits heat conduction and promotes the migration and enrichment of Li and Mg toward the surface，which forms a temperature gradient and induces a “micro-explosion” effect， shortens the ignition delay， but reduces combustion completeness and combustion intensity.