Calcium hydrogen phosphate is synthesized by using hydrobromic acid separation from waste liquid generated in the production of bromine hydride via quaternary ammonium salt phase trans-catalyst, based on preparing phosphoric acid by wet process. 将采用季铵盐相转移催化剂生产溴化氢时产生的废液中的氢溴酸分离出来,在制取湿法磷酸的基础上合成出磷酸氢钙。