The effects of nano CaCO3 (NC) and fly ash (FA) on the slump, strength, hydration reaction characteristics and microstructure of recycled aggregate concrete were studied. The results show that the negative effect of slump caused by adding NC can be reduced by adding FA; The addition of NC can make up for the decrease of early strength of recycled aggregate concrete caused by the addition of FA; The increase of 90 d compressive strength of recycled aggregate concrete mixed with NC is reduced, and the 90 d compressive strength of recycled aggregate concrete is improved after they are mixed together. With the addition of NC and FA, the content of calcium hydroxide (CH) crystal in recycled aggregate concrete decreases significantly; NC can improve the hydration reaction rate of cement, refine hydration products, repair the interface transition zone (ITZ) between old mortar and new mortar, and improve the compactness of microstructure of recycled aggregate concrete.