Abstract: Based on molecular simulation technique, the cracking rule of silicon insulating oil under thermal stress and the influence mechanism of moisture were studied. Polydimethylsiloxane（PDMS） cracking path and product formation mechanism of polydimethylsiloxane monomer molecule in silicon insulating oil were analyzed by using ReaxFF reaction molecular dynamics software. The results show that the temperature increase will promote the cracking of silicon insulating oil and speed up the formation rate of various characteristic products. CH4 and H2 are the main characteristic products of silicon insulation oil cracking. The initial bond breaking position of PDMS molecule is C-Si bond at both ends of long chain molecule, and then it continues to crack to generate other characteristic products. Formaldehyde（CH2 O） is the special product of silicon insulation oil cracking, which can be used as the characteristic quantity to judge the performance deterioration of silicon insulation oil. Water promotes the cracking of PDMS. PDMS decomposes before water, which then provides H and O to the reaction system to promote the formation of characteristic products. In order to verify the correctness of the simulation, the accelerated thermal aging experiment of silicon insulation oil with different water mass fraction was carried out. The simulation results are compared with experiments to verify the validity of the simulation results.