In order to clarify the reaction process of MgO-SiO2-H2O system, the reaction products and reaction degree were characterized.Further, the reaction thermodynamics was calculated and the reaction kinetics was deduced.The results indicated that a large amount of Mg(OH)2 generated and a little magnesium silicate hydrate (M-S-H) gel formed with the dissolution of MgO.M-S-H gel was generated gradually afterward until silica fume (SF) or Mg(OH)2 was consumed completely.The formation reaction of M-S-H gel is the main reaction in MgO-SiO2-H2O system when the dosage of MgO is lower than 50 %, while the formation reaction of Mg(OH)2 is the main reaction when the dosage of MgO is higher than 50 %.Based on the thermodynamic calculations and the experimental results, M-S-H gel is more stable than Mg(OH)2 and the final products tend to be M-S-H gel rather than Mg(OH)2.Based on the measured values, the reaction kinetics of MgO follows (R2>0.97).