Abstract: By combining thin micro oxidation test and infrared spectrum analysis, the relations of the sediment content，mass reduction under the setting oxidation conditions with oxidation time were simulated and analyzed. It can be seen that the deposit appears after 90 minutes oxidation and increases with the extension of time. It is discovered that the ratio of carbonyl and methyl group infrared spectrum areas of PAO (CSI) can well reflect the depth of oil oxidation. The ratio first increases and then decreases as the oxidation time is extended. The results show that the good thermal oxidation stability of PAO lubricating base-oil can keep about 90 minutes at 230 ℃.