Red mud (RM) is an aluminum industrial solid waste with a global inventory exceeding 4 billion tons. To utilize RM efficiently, this study used RM as the primary material, with cement, steel slag powder, and desulfurization gypsum as supplements, to prepare foamed mixture lightweight soil containing RM (FMLSR) for subgrade filling. And the effect of nano-SiO2 on the fresh properties, unconfined compression strength, durability, and microscopic properties of FMLSR was investigated. The test results indicated nano-SiO2 significantly reduced the flowability of FMLSR. Therefore, to maintain suitable flowability, the nano-SiO2 content should not exceed 1.5%. When the content ranged from 1.0% to 1.5%, nano-SiO2 effectively enhanced the compression strength of FMLSR, reduced its volumetric water absorption, and improved its resistance to dry-wet cycles. Nano-SiO2 can also reduce the environmental risk of FMLSR by significantly lowering the pH of FMLSR leachate. Microanalysis revealed that nano-SiO2 enhanced the density of the skeletal structure of FMLSR by promoting the formation of ettringite, thereby improving its properties. The addition of nano-SiO2 increased the CO2 emissions of FMLSR, but as nano-SiO2 improves various aspects of FMLSR properties, this increase is acceptable. In conclusion, the suitable content of nano-SiO2 for FMLSR ranged from 1.0% to 1.5%.