To evaluate the aging performance of acrolein-pentaerythritol resins （123 resins）-RDX based casting polymer bonded explosive （PBX）， accelerated aging tests were performed according to MIL-STD-1751. The microstructure and aging mechanism of casting PBX during aging were investigated by in-situ Infrared Spectroscopy （IR）， 3D-Super Depth of Field Digital Microscope and Scanning Electron Microscope （SEM）. After aging 260 days at 65 ℃， the mass and size change rates of the casting PBX columns are both within 0.25%， which is better than the standards （<1%） of U.S. military standard MIL-STD-1751. The results indicate that PBX columns are stabilization at an acceptable level after aging tests. In addition， no obvious variations on both surface and corss-sectional morphology are observed， except for color changes. The interface between RDX particles and the binder is still well bonded， and the cross section is mainly transgranular fracture. The mechanical properties including both compressive strength and tensile strength of 123 resins-based casting PBX increases obviously with the increase of aging time. After aging 60 and 150 days at 65 ℃， the compressive strengths increase by 6.42 MPa and 13.69 MPa with the increasement of 8.46% and 18.05%， respectively. After aging 90 and 180 days at 65 ℃， the tensile strengths increase by 0.78 MPa and 1.13 MPa with the increasement of 6.34% and 9.19%， respectively. The mechanism of the mechanical properties’ increase was studied by in-situ IR method and the post-casting behavior during aging process was considered as the main reason.