Abstract: The relationship between calcium activity (α_[Ca]) and aluminum content was analyzed by thermodynamic calculation to guarantee inclusions in liquid region after calcium treatment for X80 pipeline steel, and the α_[Ca] ranges were determined to obtain liquid calcium-aluminates inclusions. The effect of relationship between[Al] and[S] on the forming of CaS and the effect of temperature on the precipitation of CaS were calculated quantificationally. The results show that when the mass percent of sulfur is 20×10^-6, the CaS solubility in calcium-aluminates can reach 0.18%.