In the environments of saturated limewater at 20±2°C and fly ash in cement mortars at 20±2°C, the hydration mechanism of fly ash is studied. The results indicate: under this test condition, the reaction speed of fly ash presents the degressive tendency basically within 90 days, and its chemical activation is an accumulation change process. The hydrated course of fly ash in the two environments is resembled, and only the age effectiveness is inconsistent. The test methods of Ka value and reaction degree can be used to analyze chemical activation of fly ash effectively. Rapid evaluation can be used on early activation of fly ash by Ka value and the later period one can be estimated by reaction degree.