The invention relates to the technical field of coating, and in particular relates to modified epoxy resin and a preparation method thereof as well as electrophoretic coating prepared from the modified epoxy resin. A molecular chain of the modified epoxy resin comprises a phosphorus-based group. The phosphorus-based group has a three-grade ion state; ionization constants of each grade of ions havedifference; the three-grade ion state of the phosphorus-based group can be used for promoting gradual deposition of a paint film when the electrophoretic coating prepared from matrix resin is subjected to electrophoretic deposition; the paint film has good density, uniformity and accumulation form, so that the appearance and the corrosion prevention property of a prepared product can be greatly improved. The hydrated ions of the phosphorus-based group are more stably dispersed in an emulsion state. The phosphorus-based group is introduced into the epoxy resin and is used as the matrix resin of the electrophoretic coating; when the phosphorus-based group is subjected to electrolytic reduction, an ion group disappears to form phosphide and an insulator is formed; the electrophoretic force of the electrophoretic coating is effectively improved.