Abstract: With 99% industrial boric acid as raw material，after the oxidation and recrystallization to remove organic impurities， esterification distillation reaction with methanol and highly pure boronic acid product with purity of 99.999% was finally obtained by hydrolysis of purified borate ester.The esterification distillation reaction was controlled at about 90 ℃ and product collection temperature was 60 ℃.The process has many advantages，such as lower temperature，easy-to-implement process conditions，well-controlled volatile impurities into the distillation product，and good purification effect.And then after dehydration at high temperature，molding，and other processes，a liquid sealant suitable for gallium arsenide crystal growth，i.e.high purity boron oxide was ultimately prepared.The important optimizations of esterification distillation and hydrolysis reaction conditions，as well as the heating dehydration process were also studied.

Key words: high-purity boric acid, esterification reaction, boron oxide, vacuum dehydration