The invention discloses a DOPO-based symtriazine ring hydrogenated benzimidazole epoxy curing agent and a preparation method thereof. The preparation method comprises the following steps: firstly, mixing a solvent I, tricyanogen chloride, p-hydroxy benzaldehyde and an acid-binding agent, introducing nitrogen gas, performing heating reaction to obtain 2,4,6-tri(4-formylphenoxy)-1,3,5-symtriazine ring; mixing a solvent II, the 2,4,6-tri(4-formylphenoxy)-1,3,5-symtriazine ring and DOPO, introducing nitrogen gas, and reacting to obtain DOPO-based formylphenoxy-1,3,5-symtriazine ring; dissolving the DOPO-based formylphenoxy-1,3,5-symtriazine ring in a solvent III, dropwise adding solute into a mixture of o-phenylenediamine and the solvent III, adding a catalyst, and reacting to obtain the DOPO-based symtriazine ring hydrogenated benzimidazole epoxy curing agent. According to the DOPO-based symtriazine ring hydrogenated benzimidazole epoxy curing agent, vertical combustion of cured matters passes UL94V-0 level; compared with non-flame-retardant epoxy resin, the glass-transition temperature is increased by 12.1% and the tensile strength is improved by 18.2%.