The leaching law of Ca2+ from steel slag before and after carbonation were analyzed by spectrophotometry with chlorophosphonazo as spectral chromogenic reagent. The content of f CaO in steel slag was tested with ethylene glycol. The results show that under carbonation conditions of 70℃, 80%RH, φco2=99.9%, pco2=0.35MPa, the leaching concentration of Ca2+ from steel slag(0.154~1.000mm) reduces from 102.31μg/mL before carbonation to 44.97μg/mL after 180 min carbonation. The content(by mass) of f CaO in steel slag(<0.074mm) reduces from 2.67% to 0.58%. The smaller the particle size of steel slag, the higher the leaching concentration of Ca2+ for the same leaching time.