The invention discloses a preparation method for a benzoxazine intermediate and a synthetic resin of the intermediate. The method comprises the following steps: adding 2 to 2.4 parts by mol of an aldehyde solution with a mass percent of 35 to 40% and 1 part to 1.2 parts by mol of primary amine compounds into 80 to 120 parts by mol of an organic solvent, controlling reaction temperature to be no more than 30 DEG C, adjusting the pH value of an obtained mixture to be 7 to 14, maintaining the mixture at a constant temperature for 30 minutes, adding 1 part by mol of urushiol, heating an obtained mixture to a temperature of 90 DEG C, allowing the mixture to react for 5 to 10 hours, carrying out vacuum distillation on a reaction solution, dissolving a product with a fat-soluble solvent, rinsing the product 3 to 5 times, carrying out drying after removal of the solvents so as to obtain the desired benzoxazine intermediate, and curing the benzoxazine intermediate in an electrothermostat at different stages. According to the invention, the considerable advantages of a simple synthetic method, easily available raw materials, low cost, low viscosity, a liquid state at normal temperature and suitability for industrial production are achieved; polymerized polybenzoxazine has excellent heat resistance and mechanical properties.