Abstract: The activity SiO2 and Al2O3 content, the vitreous phase content and the infrared spectrum analysis and other method were applied to the comprehensiv e study on the reaction activity and the strength of the fly ash under the autoclaved condition.The results showed that under the autoclaved condition, the reaction activi ty of the fly ash and the activity SiO2 and Al2O3 content, vitreous phase content as well as the Si- -O expansion and vibration frequency in the strong absorption block near infrared spectrum 1 100 cm-1 would have an obvious corresponding relationship.The higher the activity SiO2 and AI2O3 content was, the higher vitreous phase Co ntent would be and the Si- 0 expansion and vibration frequency in the strong absorption block near infrared spectrum 1 100 cm-1 would be lower.The higher autoclave d reaction activity, the higher the strength of autoclaved products would be.