Short-medium chain saturated triglyceride is a low viscosity and degradable liquid dielectric material, which has potential applications in the field of transformer internal insulation. In this paper, the molecular properties of short-medium chain saturated triglycerides under the action of electric field are studied by using density functional theory and time-dependent density functional theory. The results show that the molecular bond length is obviously dependent on the electric field intensity, which is consistent with the shift of the infrared wave number peak. Under the same electric field, with the increase of the length of carbon chain, the dipole moment and polarity of the molecule increase correspondingly. When the electric field intensity is 10⁹ V/m, the energy of the highest occupied molecular orbital of tricaprylin and tricaprin increases obviously, and the ionization potential decreases sharply. The difference in excitation characteristic between molecules is small, and the decrease of excitation energy is much smaller than that of ionization potential under the same electric field. The results are helpful in improving the understanding of discharge mechanism in ester dielectric, and provide the theoretical support for the performance improvement of natural ester insulating oil.