The concretes with recycled aggregate mass replacement rate of 30% were placed in 3%Na2SO4, 5%Na2SO4, 10%Na2SO4 (3%, 5%, 10%, mass fraction) solution and water for freeze-thaw cycles test by quick freezing method. The mass loss rate, relative dynamic elastic modulus change and compressive strength loss rate of recycled aggregate concrete were tested. The microstructure of the damaged layer of recycled aggregate concrete was analyzed by electron microscope, energy spectrometer and X-ray diffraction. The thickness of the damaged layer was determined by ultrasonic flat measurement method, and the erosion coefficient was introduced to optimize the damage degree with the thickness of the damaged layer as the evaluation index. The results show that: when freeze-thaw cycles is 0~200 times and the concentration of Na2SO4 solution is greater than 5%, the erosion coefficient of compressive strength is always less than 1, in other ways, Na2SO4 solution has obvious promoting effect on the macroscopic mechanical performance damage of recycled aggregate concrete, but has obvious inhibitory effect on the microstructure damage. In the early stage of freeze-thaw cycles of recycled aggregate concrete, freeze-thaw erosion is the main factor. In the later stage of freeze-thaw cycles, sulfate chemical erosion is the main factor. After chemical erosion, expansion products such as ettringite and gypsum are formed in recycled aggregate concrete, and they lead to expansion cracks. The cracks expand rapidly under the action of freeze-thaw cycles, and the thickness of the damaged layer increases. The accuracy of the damage degree based on the thickness of the damage layer as the evaluation index is improved by at least 26.33% after optimization.