In this paper, a sealed in-situ reaction system and infrared spectroscopy (IR) were used to study the gas releasing mechanism of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) at different temperatures. Three characteristic stages of LLM-105 during the heating process to 400 degrees C were detected by the moving-window two-dimensional correlation spectroscopy (MW2D). Two-dimensional correlation infrared spectroscopy (2DCOS) was employed to examine changes of gas concentration in the sealed system at each stage. We found that in the first stage, LLM-105 first released H2O and NH3. In the second stage, the decomposition is intensified, the explosive molecule first undergoes the decomposition reaction of releasing H2O, and CO2, subsequently chemical reaction producing several nitrogen gases, such as NO, N2O, HCN, NO2 and NH3. In the third stage, the decomposition process mainly releases CO2 and NH3, and the concentrations of HCN and NO2 decreased gradually. 2DCOS proven to be a very sensitive method for detecting thermal decomposition mechanisms and monitoring gas changes in sealed system.