The microbial degradation performance of rapeseed plant insulating oil (RDB), FR3 transformer insulating oil (FR3) and 25^# transformer insulating oil is studied by methods in Rapid Biodegradability:Closed Bottle Test. The results show that the degradation rates of RDB, FR3 and 25^# transformer insulating oil are 98.87%, 99.57% and 37.73%, respectively. According to the New Chemical Hazard Assessment Guidelines (HJ/T 154-2004), RDB and FR3 are biodegradable chemicals, while 25^# transformer insulating oil belongs to chemicals difficult to biodegrade. The degradation difference between plant and mineral insulating oil is mainly because the types of constituent substances and the content of unsaturated chemical bonds are different. As the research on the degradation difference of different kinds of plant insulating oil is very limited, the degradation mechanisms of different kinds of plant insulating oil are revealed from the perspective of hydrolysis kinetics in this paper.