In order to study the effect of high pressure from screw pumping and medium deep hole charging on the microstructure and thermal stability of the on-site mixed emulsion explosive matrix， the microstructure， particle size distribution， crystallization content， thermal decomposition process， thermal decomposition reaction activation energy， thermal decomposition mechanism function and rate equation of the matrix under atmospheric and high pressures were studied by optical microscope， laser particle size analyzer， water solubility experiment， thermogravimetry and derivative thermogravimetry （TG-DTG） couple method， Kissinger method and Ozawa method， Coats-Redfern method and Šatava method. The results show that from atmospheric pressure to high pressure， polymerization， demulsification and crystallization of the intra-matrix phase droplets appeared， the particle size increased from 3.717 μm to 4.474 μm， the precipitation amount of ammonium nitrate crystals increased from 0.0530 g to 0.0640 g， and the uniformity of the emulsion system was weakened. The average thermal decomposition onset temperature of the matrix T_onset increased from 157.4 ℃ to 184.0 ℃， the average first-order derivative thermogravimetric peak temperature T_p increased from 262.6 ℃ to 281.8 ℃， the average mass loss rate increased from 0.1454 %·s^-1 to 0.1476 %·s^-1， and the reaction activation energy decreased from 108.49 kJ·mol^-1 to 84.74 kJ·mol^-1. The free water released by evaporative demulsification under high pressure might cause the rise of T_onset and T_p， and the thermal decomposition reaction was more likely to occur. The activation energy calculated by the Ozawa method had a different trend with the increase of conversion rate， and the thermal decomposition reaction mechanism function changed from Valensi equation to inverse Jinder equation and the rate equation also changed. The high pressure promotes the process of droplet polymerization， demulsification and crystallization of the intra-matrix phase， reduces the activation energy of the thermal decomposition reaction， and weakens the homogeneity and thermal stability of the system.