Abstract: Commercial epoxy resin is mainly made from non renewable petroleum-based raw materials, and it has the problem of flammability and non self extinguishing, which limits its widespread application in fields such as electronics, electrical engineering, and rail transit. Therefore, a novel epoxy oligomer with magnolol phosphaphenanthrene (ED-0.5) was successfully synthesized with biomass compound magnolol, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and epichlorohydrin, and then a high flame retardant epoxy resin (ED-0.5/DDM) was prepared by using 4,4'- diaminodiphenylmethane (DDM) as the epoxy curing agent. The chemical structure was characterized by FTIR and NMR, and the curing kinetics and thermal stability of epoxy resin were studied by DSC and TGA. The flame retardancy of epoxy resin was investigated by limiting oxygen index, vertical combustion, and cone calorimeter. The thermal decomposition gas phase composition of the cured resin and the morphology of the carbon layer after combustion of the epoxy resin were analyzed by TG-FTIR and SEM, respectively, to study its flame retardancy mechanism. The results show that the thermal stability and thermal mechanical properties of ED-0.5/DDM epoxy resin are better than traditional bisphenol A epoxy resin, but the activating energy of the curing reaction is higher. The limiting oxygen index (LOI) of ED-0.5/DDM epoxy cured resin can reach 52.5%, passing the V-0 level of UL-94, reducing the peak heat release rate and total heat release by 70.9% and 58.2% respectively, and increasing the residual carbon by 1565.2%. ED-0.5/DDM epoxy resin can capture and quenching active radicals, generate a large amount of non-burning gases, form a dense carbon layer, and exert the synergistic flame retardant effect of gas phase and condensed phase.