The measured temperature-rise was high and the lasting time of highest temperature was long in the RCC of 3rd phase cofferdam of TGP, which, in addition to other factors, was related to hydration heat of fly ash. A so called separation method was employed to calculate the hydration heat of fly ash in RCC at ages up to 150 days. The hydration heat of fly ash is about one third of that of moderate heat Portland cement at 28 days and a half of that at 150 days. Considering the modified coefficient using 0.6 water-cement ratio, the heat producting in RCC may be effectively decreased. For massive RCC under bad heat dissipation conditions, the hydration heat of fly ash, especially in the later ages, should not be ignored.