The combination of low-heat cement and fly ash is suitable as a cementitious material for dam concrete. In this work, the hydration heat release, hydration products and hydration mechanism was investigated by the experiments and model analysis. The results show that the incorporation of fly ash has a significant impact on the hydration process of cement, including delaying the hydration time and significantly reducing the hydration heat. Due to the volcanic ash effect of fly ash, the calcium hydroxide will be consumed. When the amount of fly ash increases, the volcanic ash effect becomes more pronounced. The amount of calcium hydroxide increases first and then decreases with time, and the largest is at 28d hydration. The proposed model were adopted to simulate the C-S-H, calcium hydroxide, ettringite, etc. The model results basically consistent with the experiment results. The combination model is an reliable analytical tools to study the hydration of low heat cement blended with fly ash.