The process of preparing boron-containing benzoxazine resin includes the following steps: 1. preparing materials including boric acid 20-70 weight portions, phenolic compound 80-240 weight portions, amine compound 40-120 weight portions, water solution of formaldehyde 160-360 weight portions, acid catalyst 1-4 weight portions and alkaline catalyst 4-8 weight portions; and 2. reaction of boric acid, phenolic compound and acid catalyst in a reactor, and heated reflux dewatering to obtain borate; heating to 50 deg.c, adding partial formaldehyde to synthesize boron-containing linear phenolic resin; adding alkaline catalyst to regulate pH to 6-9, adding amine compound and the rest formaldehyde to reflux react at 85-95 deg.c for 2-8 hr, and vacuum dewatering or reflux dewatering at 80-140 deg.c to obtain the boron-containing benzoxazine resin. The resin is used in producing electrically insulating material and product in high flame retarding level.