The effect on the kinematic viscosity of poly-α-olefin (PAO) aviation lubricating base oil under high temperature by two kinds of antioxidants, 2,6 ditbutyl-p-cresol (DBPC) and p, p'-diisooctyl diphenylamine (DODPA), was studied using the autoclave to simulate the actual operating environment of lubricating oils. The composition of constitution and structure of oil samples with different viscosities were analyzed by integrated techniques of FTIR and GC/MS from the molecular level. The results show that high temperature pyrolysis is the main reason resulting in the viscosity decay of PAO, and the more the produced low molecular weight compounds in the oil samples, the more severe is the viscosity decay. Adding of the antioxidants can inhibit the viscosity decay of PAO effectively, and has good effects of antioxidation, among which DBPC shows a more excellent high temperature oxidation resistance than the DODPA. By using an autoclave to simulate actual operating environment of lubricating oils, analyzing the change of the lubricating oil viscosity by means of physical and chemical analysis method, and adopting FTIR and GC/MS to study the change of the molecular structure of lubricating oils, the method is ideal to study the decay of oil in using and master the oil change period accurately.