The invention discloses a method for synthesizing nuclear-grade boric acid by hydrolyzing boron trifluoride, which comprises the following steps: (1) reacting boron trifluoride with boron 10 abundance of more than 96% with water, setting nitrogen or other inert gases as shielding gas or not, setting the molar ratio of boron trifluoride to water to be 10: 1-1: 100, the initial reaction temperature to be-20 to 100 DEG C, and the reaction time to be 0-40 hours, collecting a boric acid aqueous solution at the bottom of the reaction kettle, and absorbing generated hydrogen fluoride with water or alkali liquor; and (2) recrystallizing and purifying the boric acid obtained in the step (1), setting the crystallization temperature to be 0-50 DEG C, and setting the solution concentration to be 0-50%. The method disclosed by the invention is a process capable of directly obtaining the boron 10-enriched nuclear grade boric acid, the raw materials are easy to obtain, the process waste is less, and remarkable economic and environmental benefits are brought; and high temperature and high pressure are not needed, and energy waste is not caused, so that the overall cost of the process is reduced, the purity and yield of the product are improved, the purity of the synthesized boron-10-rich boric acid can reach 99.9%, the abundance of boron-10 is greater than 96%, and the boron-10-rich boric acid can be used for nuclear power station neutron protection.