The reaction between mullite and carbon in char between 1200 and 1500 degrees C during coal gasification was investigated by XRD, Fact Sage, and molecular simulation. The mechanism of thermal reaction is discussed by both thermal dynamic and molecular simulation methods. The effect of carbon thermal reaction on the char gasification was also discussed. The results show that the carbon thermal reaction between mullite and carbon in char only occurs in argon above 1300 degrees C and the mullite could react with carbon completely to form moissanite (SiC) and corundum (Al2O3) at 1500 degrees C in argon. This reaction is restrained by the existence of CO, and it occurs between 1300 and 1500 degrees C when the partial pressure of CO in the atmosphere is less than 0.4 according to results of the thermodynamic calculation. In hence, carbon thermal reaction only happens in two following situations during gasification. One is the melted ash or slag over the surface of the char and the other is that the char particle is wholly covered by melted slag. The moissanite reacts with CO2 to form SiO2 and release CO during char gasification. The carbon thermal reaction between mullite and carbon in char facilitate the high-temperature coal gasification and could reduce the content of unreacted carbon in slag.