The invention relates to a method for preparing high-purity ammonia from residual ammonia water. The residual ammonia water is taken as raw material, and the high-purity ammonia of which the purity is 99.8% is prepared through the processes such as residual ammonia water distillation, ammonia vapor condensation and cooling and ammonia water pressurized distillation. A coke oven flue gas desulfurization and denitrification device is provided with the required raw material ammonia while the residual ammonia water is treated.