By testing the content of free-CaO in the sample,the content of free-CaO in the sample was regarded as the measurement of reacting.The influence of MgO on the formation of mineral of C_2S which is the component of highC_3S cement was studied by the research methods of chemical analysis and thermal analysis.It is found that appropriate adding of MgO helps absorption of free-CaO,and benefited the formation of mineral of C_2S;furthermore,lowers decomposition temperature of CaCO_3.At last,regarding the thermodynamic reason of this phenomenon,it is found that adding MgO increases the ratio of n(CaO)/n(SiO_2),enhances the thermodynamic function of C_2S mineral formation.