Optimizing the mix ratio of RCC (rolled compacted concrete) is an important technical measure to protect dam safety. With RCC dam project in cold area as an example, we discuss the dosage of admixture and quality of fly ash affecting the mix proportion design, and obtain the fitted relationship curves between RCC binder-water ratio and compressive strength under different working conditions. Results show that increasing the dosage of admixture could effectively reduce the plastic material dosage without affecting the quality of roller compacted concrete; fly ash of different grades has great effect on the water use of concrete; the water-binder ratio has good linear correlation with compressive strength. The variation of compressive strength development coefficient leveled off after the first increase.