Preparation of α-calcium sulfate hemihydrates (α-HH) from phosphogypsum (PG) under atmospheric pressure has been a promising way to utilize PG. In this study, the phase transition from PG (the main composition is calcium sulfate dihydrate, DH) to α-HH is investigated in NaCl solutions with different concentrations and reaction temperatures under atmospheric pressure. The results show that the concentration of NaCl affected the phase transition as well as the crystal morphology. Higher reaction temperature above 85°C is found to accelerate the phase-transition reaction and the products are stable in 18.8 wt% NaCl solution, which is resulted from decreasing water activity with increasing NaCl concentration. The phase-transition product of PG is a mixture of α-HH and omongwaite (Na2Ca5(SO4)6·3H2O). Differential scanning calorimeter and thermogravimetry curves and chemical composition, especially Na2O content variation, suggest a third phase, omongwaite is formed during the transition from PG to α-HH.