By means of autoclave device to simulate the actual working conditions of PAO,the relationship between PAO performance recession and its structural change was analyzed by GC/MS modern detection technology from the molecular level.The results show that the thermal cracking at high temperature is the main reason of PAO deterioration,and the performance wouldn’t change a lot at normal working temperature until the temperature is more than 230 ℃.PAO branched molecules break at high temperature to generate a lot of small molecule hydrocarbons,resulting in a decline of viscosity and a rise of pour point.Only a small amount of oxygen containing compounds is detected,and acid value change is very small.Structure decides to use performance,so the thermal stability of PAO with the branched chain is poorer than its oxidation stability.