Abstract: There is large quantity of waste liquor produced，after extracting magnesium in the production of magnesium hydroxide with lime-brine method.In this kind of liquor，there contains calcium chloride，sodium chloride and potassium chloride etc.The sideline product calcium sulphate will be produced by the mixing of brine and waste liquor.The various factors affecting the chemical transform from Ca↑2+ to calcium sulphate and the purification for the resultant of reaction (i.e.calcium sulphate dihydrate) were studied.Based on the research results，the industrial operating conditions were confirmed and which   have been put into practice.The new technology not only realized the full utilization of the waste liquor after extracting magnesium，but also provided favorable conditions to the comprehensive utilization of brine and salt-manufacturing，especially for the extraction of potassium chloride.