Direct phase transformation of flue gas desulfufization gypsum in hot salt solution at atmospheric pressure was investigated. The effects of temperature, salt species, salt concentration, solids content, pH and modifier were examined. The crystals obtained under different conditions and solubility of calcium sulfate in contact with solid gypsum were also determined, alpha-Calcium sulfate hemihydrate crystals of stubby columnar shape and regular pentahedral sides were obtained under the following conditions: salt concentration 20%-30%, operation temperature 95-100 degrees C, solids mass content in the slurry 10%-30% and neutral pH. Thermodynamic analysis revealed that phase transformation of calcium sulfate dihydrate to alpha-calcium sulfate hemihydrate occurs because of the difference in solubilities between the two solid gypsum phases in this system.