Non evaporable water test, mercury intrusion porosimetry(MIP), scanning electron microscopy(SEM) observation and mechanical properties test were used to study the hydration degree, microstructure and mechanical properties of ultrafine fly ash and cement cementitious materials under curing at 50℃ to compare the property of ultrafine fly ash with that of ordinary fly ash. Research results show that, compared with the ordinary fly ash, under the mechanical grinding and high temperature curing at 50℃, the ultrafine fly ash and cement cementitious materials has faster hydration rate, finer pore size distribution and higher compressive strength and flexural strength. When the content of ultrafine fly ash is 25%，the hydration degree and pore structure compactness of the composite cementitious material are higher than that of pure cement paste, and the compressive strength and flexural strength of composite cementitious material at 3d are nearly twice higher than that of pure cement mortar, the longer the age, the more obvious the effect of improving the strength. However, when the content of ultrafine fly ash is 50%, the concentration of cement in the system is greatly reduced, because of the large amount of fine aggregate, the particle distribution is unreasonable, and the activity of ultrafine fly ash is reduced due to the lack of sufficient Ca(OH)2. Therefore, the degree of hydration, the compactness of the pore structure, the compressive strength and flexural strength of the composite cementitious materials containing 50% ultrafine fly ash is lower than that of containing 25%, and the flexural strength at 7d still reaches 167 times that of pure cement mortar.