Natural ester insulating oil has the advantages of high fire point, renewability and good biodegradability. Compared with mineral insulating oil, natural ester insulating oil has the problem of high dielectric loss factor, which affects the use of natural ester insulating oil in transformers. In order to reduce the dielectric loss factor of natural ester insulating oil and take into account the basic properties, transesterification-modified palm oil-based trimethylolpropane ester insulating oil was prepared, and the influences of molecular structures on the polarization characteristics of natural ester insulating oil were studied by quantum chemical calculation. The results show that the polarization characteristics of natural ester insulating oil are closely related to its molecular structure and functional groups, and the dielectric loss of plant insulating oil can be reduced by increasing the carbon number of lipid chains and reducing the content of C=C double bonds. Based on quantum chemical calculations, palm oil was selected as the basic unit, and the prepared palm oil-based modified polyol esters had a low dielectric loss factor of only 0.4%.