Mullite precursor was prepared from kaoline when calcined at 1250 °C，when the concentration of sodium hydroxide of 3 mol/l; crystallization temperature of 80 °C; holding time of 3 h and precursor concentration of 0.15 g/ml, the morphology of the prepared mullite nanocomposite appears spherical-like, and it has good crystallization property and low agglomeration, and the average size of the particles is about 60.9 nm. Both the thermodymics and kinetics of mullite precursor have been studied. The results are as follows: the reaction started when the calcine temperature T﹥495°C; mullite precursor formed when the reaction system temperature 1000 °C; the activation energy of precursor Ea=928.3 KJ/mol; the reaction order n=1.33. We have reported our successful experiments to use kaolin to synthesize mullite nanocomposite with satisfying, spherical morphology, relatively homogenous grain average size of 60.9 nm, and little aggregation[1-2]. This paper presents an investigation of the thermodynamics and kinetics of the precursor preparation process.