The invention provides a method for preparing a neodymium iron boron magnet through grain boundary diffusion. The method comprises the steps that appropriate basic neodymium iron boron is selected, abasic neodymium iron boron magnet is cut into sheets with the thickness smaller than 8 mm to obtain neodymium iron boron sheets; the neodymium iron boron sheets are soaked in a hydrochloric acid solution or nitric acid solution for 5-20 minutes and then are dried in the environment of 120-140 DEG C for 20-120 minutes; an evaporation method is utilized to attach a layer of heavy rare earth metal tothe surfaces of the dried neodymium iron boron sheets; the neodymium iron boron sheets with the surfaces evaporated with the heavy rare earth metal are firstly subjected to vacuum infiltration underthe condition of 750-790 DEG C for 1-2 hours and then is subjected to continuous vacuum infiltration treatment under the condition of 951-1000 DEG C for 2-4 hours; the neodymium iron boron sheets obtained after vacuum infiltration are subjected to vacuum ageing under the condition of 500-600 DEG C and then are subjected to vacuum ageing under the condition of 400-450 DEG C to obtain a target product. By adopting the preparation method, the influence on residual magnetism is small while the coercivity of the magnet can be improved.