<p>The growing population and rapid urbanization have led to an increase in generation of solid wastes. Utilization of these wastes as backfilling materials promotes sustainability because it mitigates landfilling of solid wastes, produces valuable backfilling materials, and conserves natural resources. Hence, this study proposes a solution to use two solid wastes, fine incineration bottom ash (FIBA) and marine clay, to produce liquefied stabilized soil (LSS) for backfilling. The influential factors including initial water content of marine clay slurry, cement content, and FIBA content in the production of LSS were investigated and discussed. The flowability, water content, void ratio, elastic modulus, unconfined compressive strength (UCS), leaching concentrations of heavy metals, mineralogy, and microstructure of the LSS were evaluated. The costs and benefits of the proposed LSS were also analyzed. The results showed that FIBA affected the flow value of LSS through decreasing water content and cation exchange. The addition of FIBA not only reduced water content and void ratio of LSS, but also promoted formation of hydration products such as calcium silicate hydrate and ettringite, thereby increasing the strength of LSS. LSS with a cement content of 60 kg/m³ and a FIBA content of over 200 kg/m³ achieved a similar or even higher UCS than LSS with 80 kg/m³ of cement but without FIBA, demonstrating its potential for cement reduction. The leaching concentrations of heavy metals from LSS were considerably reduced compared with raw FIBA due to the sorption by clay minerals and the stabilization/solidification by hydration products. The cost–benefit analysis indicated that compared to commercial LSS, producing LSS using only marine clay reduced material costs by 91%, while incorporating FIBA can further generated revenue through waste recycling fees. In summary, the proposed eco-friendly waste-based LSS possesses good engineering properties and improves cost-effectiveness for backfilling applications.</p>