Foam lightweight soil (LS) is a cement composite with excellent lightness, but the excessive use of cement causes some negative impacts on the surrounding environment. This study aims to develop a sustainable cement composite by utilizing fly ash and waste soil in LS, providing a practical reference for green construction of road engineering. The physico-mechanical properties of cement composites with different mixing ratios were comparatively evaluated using geotechnical tests, and the micro-mechanisms were investigated using microscopic tests. The testing results showed that the utilization of fly ash and waste soil was unfavorable to improve the mechanical strength and the damage resistance of LS, but significantly decreased the use of cement. The comprehensive performance of cement composite reached the optimum when the replacement rates of fly ash and waste soil were 10% and 20%. Fly ash reacted with the hydration products of cement producing more cementitious gels to make the internal structure of cement composite denser, while waste soil not involved in its chemical reaction. The life cycle assessment indicated that the potential environmental impact of LS was improved after utilizing fly ash and waste soil, and the proposed sustainable cement composite had good feasibility in engineering.