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 betweenAl andS 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%.