[Purposes] The surface of steel slag aggregate was modified by cement-fly ash composite slurry. The evolution law and deterioration mechanism of macroscopic mechanical properties of modified steel slag aggregate concrete under sulfate were then investigated. [Methods] The corrosion test of modified steel slag aggregate concrete under sulfate action was carried out, the evolution laws of the mass loss rate, dynamic elastic modulus and compressive strength of the modified materials were revealed. The microscopic characteristics of modified steel slag aggregate concrete under sulfate attack were explored by X-ray diffraction (XRD) and scanning electron microscopy (SEM). [Findings] The quality and dynamic elastic modulus of modified steel slag aggregate concrete increased first and then decreased with the erosion time. Compared with steel slag aggregate concrete and ordinary concrete,the compressive strength of modified steel slag aggregate concrete increased by 5.7% and 15.6% respectively at 56 d. The modified steel slag is wrapped by cement-fly ash slurry, alleviating the damage process in the process of sulfate attack. The internal erosion products of the modified steel slag aggregate concrete was significantly reduced. At the same time, the modified steel slag contains active components that can undergo hydration reaction, the pores and cracks will be effectively filled by the hydration products generated. Hence, the corrosion resistance of the modified steel slag aggregate concrete is significantly improved. [Conclusions] The performance of steel slag aggregate concrete can be modified by the cement - fly ash slurry,and the stability is improved, the sulphate resistance performance was effectively enhanced. The research can provide a reference for the application and promotion of modified steel slag.