Boric acid and kieserite were prepared from low-grade ascharite by sulfuric acid method. This method results in the recovery of 71.06% and 45.03% for boric acid and kieserite, respectively. Meanwhile, the boric acid was precipitated from the filtrate at low temperature and the solution was recycled without discharging waste liquid in the whole process. The influence of amount of sulfuric acid, mass fraction of sulfuric acid, reaction temperature and reaction time on the leaching rate of boric acid were studied. The results show that the leaching rate of boric acid reaches 93.80% under the following conditions: the amount of sulfuric acid is 85% of theoretical dosage; the mass fraction of sulfuric acid is 25%; reaction temperature is 95 ℃; and the reaction time is 100 min. Meanwhile, the effects of mass fraction of magnesium sulfate, crystallization temperature and crystallization time on the crystallization of kieserite were investigated and the optimal crystallization conditions are obtained: the mass fraction of magnesium sulfate is 28%; the crystallization temperature is 180 ℃ and the crystallization time is 4 h.