The external desulphurization of molten iron has become an important step in the production of steel and iron. The melting temperatures of high calcium desulfurization slag, which were taken from Shougang Shuicheng Iron and Steel (Group) Co. Limited, were investigated on basis of the method of hemisphaerium temperature. The contents of CaO, Al2O3 and CaF2 in the slag were adjusted in 47%-53%, 14%-20% and 2%-10% respectively by adding analytical reagent Ca (OH)2, Al2O3 and CaF2 using orthogonal design. The results show that the melting temperature of desulfurization slag decreases obviously with the increase of the content of Al2O3 between 14% and 20%, and decreased markedly with the increase of the content of CaF2 lower than 8%. The impact to the melting temperature of the high calcium slag is as follows: Al2O3 > CaF2 > CaO.