Fluidized solidified soil, due to its high fluidity, self-compaction, and controllable strength, can solve filling engineering problems such as narrow backfill sites and narrow backfill spaces. Fluid solidification materials mainly include cementitious components and flow components. This article mainly studies the cementitious material ratio and mechanism of fluidized solidification sludge. Using slag and cement as basic cementitious materials, the appropriate dosage of activator quicklime and sodium silicate was determined through double mixing experiments. Response surface analysis was conducted using Design Expert software to obtain the optimized ratio of composite cementitious materials; Obtain the types and microstructure of solidified sludge products through XRD and SEM experiments, and analyze the solidification mechanism. The results showed that the optimized ratio of composite cementitious material (CCM) cement: mineral powder: quicklime: sodium silicate was 38:42:9.7:6.5. The flowability and strength of the fluidized solidified sludge using CCM were better than those using only cement. The hydration products of CCM solidified sludge mainly include hydrated calcium silicate gel and ettringite. The hydration products encapsulate soil particles and fill pores, significantly improving the strength of fluid solidified sludge. The strength improvement of CCM solidified sludge includes hydration, alkali induced volcanic ash reaction, ion exchange, micro expansion and filling effects.