A method for preparing a boron fertilizer, including: (1) heating boric acid to a temperature of 180-200° C., maintaining the temperature for 20-30 min for dehydration of the boric acid to yield pyroboric acid； and (2) cooling down the pyroboric acid to a temperature of 40-60° C., crushing, and screening to yield a powdered, weakly acidic, high-content boron fertilizer. The method is energy-saving, environmentally friendly, and low in cost. The resulting boron fertilizer is weakly acidic, fast in dissolution rate, and has excellent in compounding performance
