To realize the recycling of magnesium sulfate wastewater in the hydrometallurgy and separation process of Baotou mixed rare earth (RE) concentrate, the research on preparation of magnesium bicarbonate (Mg(HCO₃)₂) solution from magnesium sulfate solution by CO₂ carbonation, extraction separation and transformation of RE sulfate solution by saponified HDEHP was investigated. The key influencing factors and carbonation mechanism in the preparation of Mg(HCO₃)₂, and the distribution of RE, calcium and magnesium elements in the saponification extraction were systematically studied. The results show that the conversion of Mg(HCO₃)₂ during carbonation is inhibited by calcium sulfate, but promoted by increasing the concentration of magnesium sulfate and decreasing the carbonation temperature. Under the optimized conditions, Mg(HCO₃)₂ solution with impurities (such as Fe, Al and Si) of less than 15 mg/L is prepared stably and efficiently, and the conversion rate of Mg(HCO₃)₂ is more than 95%. After multi-stage cascade extraction of RE sulfate solution, the RE extraction recovery is greater than 99.5%. Calcium sulfate crystals do not precipitate during the process, and the raffinate phase is magnesium sulfate solution that is used for the preparation of Mg(HCO₃)₂ solution.