Abstract: In order to know the influence of activity of calcium  oxide（CaO） on solid state sintering reaction at different calcining conditions， new-ecology CaO and sandrock were sintered at three different temperatures of 1 250，1 300，1 350 ℃ in experiment.Influence of activity of CaO on subsequent reaction was analyzed through the exploration to the influence of activity of CaO on free calcium oxide.Results showed that pre-calcining had apparent effect on reducing free CaO content in the product， because it could make limestone get more activity to participate solid state reaction.The higher of the CaO activity，the lower of free CaO content and the more complete of solid state reaction.

Key words: active calcium oxide, solid state sintering, free calcium oxide