In order to research the advantages and disadvantages of the ores used to make slag which have different grain sizes, six kinds of calcium carbonate minerals which have different grains are studied by X-ray diffraction, mercury, scanning electron microscopy and other analytical methods. It analyzes the grain size, the extent of fragmentation, fragmentation processes of the ore, and the ratio of pore volume, the bulk density, porosity, average pore size, activity and other factors of the lime. It researches the effects of grain size of the ore on its thermal cracking and microstructure parameters, activity of lime which is calcined. It concludes that the larger the grain size of the ore, the higher activity and more porous of lime which is calcined. When heated rapidly, the ore which has larger grain size will burst and slagging faster.