In order to address the issues of elevated water content and diminished strength of municipal sludge in the landfills, a synergistic method using ferrous sulfate and cementitious materials (such as lime, fly ash, and cement) was proposed for the in-situ solidification of landfilled sludge, with its reagent dosage further optimized by the response surface method (RSM). In this study, the water content and unconfined compressive strength (UCS) of the solidified sludge after 28 days of curing were selected as evaluation indexes. Initially, a single factor test was conducted to explore the roles of ferrous sulfate and cementitious materials in the sludge solidification process, followed by the analysis of interactions between ferrous sulfate and cementitious materials using RSM to optimize the material dosages. The results showed that both ferrous sulfate and cementitious materials could reduce the water content of the solidified sludge, with lime and cement exhibiting more significant reduction effects; the UCS of the solidified sludge increased with increasing ferrous sulfate and cement dosages, first increased and then decreased with lime dosage, and remained essentially unchanged with fly ash dosage; the quadratic polynomial model derived from RSM could accurately predict the water content and UCS of the solidified sludge, and verification indicated that the relative error between the predicted and experimental values was less than 20%; under the constraints set in this study, the water content of the solidified sludge in the recommended solution R7 (ferrous sulfate 8%, lime 6%, fly ash 10%, and cement 11%) decreased from 360.3% to 131.5%, while the UCS increased from 0 to 317.0 kPa. The aforementioned results met the requirements for landfill disposal and mechanical access.