The influence of nano TiO2(NT) on the early mechanical properties of fly ash cement system was investigated, and the mechanism was explored. The results show that the incorporation of NT can considerably improve the early compressive strength of the fly ash cement system and the optimum content of NT is 3%. In addition, NT can significantly enhance the hydration degree of cement clinker and the secondary hydration degree of fly ash, it can increase the polymerization degree of calcium silicate hydrate（C S H）gels, decrease the porosity and total pore volume, improve the pore size distribution, and thus increase the compactness and early compressive strength of the fly ash cement mortars.