Abstract: In the process of magnesium extraction from concentrated seawater,Ca<SUP>2+</SUP> is the main factor affecting purity and quality of magnesium hydroxide.Ca<SUP>2+</SUP>removal by sodium carbonate method was studied in pretreatment process of magnesium extraction.Effects of operation conditions including separation method,addition amount of sodium carbonate,stirring speed,aging time,and reaction time on removal efficiency of Ca<SUP>2+</SUP>and on loss rate of Mg<SUP>2+</SUP>were investigated.Optimum operation conditions were confirmed as follows:reaction temperature was 20~25 ℃,addition amount of sodium carbonate was 1.3 times of theoretical value,reaction time was 4 h,aging time was 48 h,stirring speed was 200 r/min,and separation method was suction filtration.Under above conditions,removal rate of Ca<SUP>2+</SUP>was more than 60% and this method effectively solved the problem that the over-high content of calcium in magnesium hydroxide paste product prepared with concentrated seawater. 