The effect of salts on the formation of ${alpha}$-hemihydrated gypsum at boiling temperature under atmospheric pressure was studied by the solubility measurement method, and the formation of ${alpha}$-hemihydrated gypsum from by-product gypsum of phosphoric acid process in the salts solution were investigated. The order of catalytic effect of salt on the formation of ${alpha}$-hemihydrated gypsum are as follows: NH4Cl>NaCl>NaNO3. In the salts solution of sodium nitrate, sodium chloride, and ammonium chloride, prismatic ${alpha}$-hemihydrated gypsum was obtained and the crystal form was converted to needle form in complex solution with sulfuric acid. The P2O5 content in gypsum was largely decreased in this atmosphoric solution process.