The breakdown of insulating oil under the lightning impulse is crucial for oil-immersed transformers. Lightning breakdown in insulating oil is typically explained by the streamer theory which is related to the ionization properties of molecules. Therefore, Density Functional Theory (DFT) was used to simulate the ionization potential (IP) of oil molecules, and the finite element method (FEM) and experimental were used to study the impact of IP differences on streamer development and breakdown velocity under lightning impulse. It can be seen that in mineral oil (MO), the molecules are less likely to ionize under electric fields. While in natural ester (NE), IP significantly decreases under electric fields, making it easier for streamer branching to occur and the breakdown velocity increases under lower voltage conditions than MO. The IP of PFAE changes with the electric field in a manner similar to MO. The development of streamer is not as significant as NE.