In order to solve the problems of high water content and low strength of municipal sludge in the landfill, a synergistic method using ferrous sulfate and cementitious materials (such as lime, fly ash, and cement) was proposed to solidify landfilled sludge in-situ, and its reagent dosage was further optimized by the response surface method (RSM). The results showed that both ferrous sulfate and cementitious materials could reduce the water content of solidified sludge, but the reduction effect of lime and cement was more significant; the unconfined compressive strength (UCS) of solidified sludge increased with ferrous sulfate dosage and cement dosage, but first increased and then decreased with lime dosage, and basically did not change with fly ash; the quadratic polynomial model obtained by RSM could accurately predict the water content and UCS of solidified sludge, and it was verified that the relative error between the predicted value and the experimental value was less than 20%; under the constraints set in this study, the water content of solidified sludge in the recommended solution R7 (ferrous sulfate = 8%, lime = 6%, fly ash = 10%, and cement = 11%) reduced from 360.3% to 131.5%, and UCS increased from 0 to 317.0 kPa, which met the requirements of landfill disposal and mechanical entry.