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, and fluidized solidified materials mainly include cementitious components and flow components. Using slag and cement as basic cementitious materials, the appropriate dosage of the activators (quicklime and sodium silicate) were determined through single factor tests. Response surface analysis was conducted using Design Expert software to obtain the optimized ratio of composite cementitious materials. The types and microstructure of solidified sludge products were obtained through XRD and SEM experiments, and the solidification mechanism of composite cementitious materials was analyzed. The results showed that the optimized ratio of composite cementitious material (CCM) cement∶slay∶quicklime∶sodium silicate was 38∶42∶9.7∶6.5. The fluidity 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 fulidized solidified sludge. The strength improvement of CCM solidified sludge includes hydration, alkali induced pozzolanic reaction, ion exchange, micro expansion and filling effects.