Ultrafine 2，6-diamino-3，5-dinitropyrazine-1-oxide （LLM-105） possesses high safety performance and low impact initiation threshold， showing promising applications in the initiation sequence. Solid-phase ripening is the main aging behavior of ultrafine LLM-105 during the storage process， which leads to the growth of particle size and performance degradation. Temperature and humidity are important environmental factors that affect solid-phase ripening， but the microscopic mechanism is still unclear. In situ small angle X-ray scattering （SAXS）， Scanning electron microscope （SEM） and in situ Atomic force microscopy （AFM） were used to capture the structural evolution of ultrafine LLM-105 particles under different temperature and humidity environments， and the solid-phase ripening mechanism was analyzed. Ultrafine LLM-105 had obvious solid-phase ripening after 30 days at 120 ℃， and its specific surface area （SSA） decreased by 41.6%. The ripening mechanism was dominated by Ostwald Ripening （OR）， accompanied with Smoluchowski Ripening （SR）. Humidity significantly accelerates the solid-phase ripening of ultrafine LLM-105 by promoting OR. After 30 days at 60 ℃ and 90% relative humidity， SSA decreased by 35.8%.