Based on the brief analysis and summary of previous research achievements, this paper put forward a method to improve the durability of medium-low-strength concrete (such as concrete used for bridge substructures). The performance including strength, chloride ion penetration resistance, sulfate corrosion resistance, anti-permeability, and frost resistance grade of such concrete with nano-modified fly ash was investigated. The results showed that when the dosing of modified fly ash was 6%, the compressive strength of 28 days was increased by 19.0% and the permeability coefficient of chloride ion was decreased by 79.9%, and the impermeability grade reached P10. With the increase of a single amount of nano-modified fly ash, the improvement of cement concrete performance is more obvious.