High-water content dredged sludge from waterways, with potential for sustainable use as high-performance fillers, was effectively treated using the vacuum preloading-flocculation-solidification combined method (denoted as the VP-FSCM). This study investigated the effect of flocculant and curing agent dosages on the solidification of sludge with initially poor mechanical properties. Ground granulated blast-furnace slag (GGBS) and ordinary Portland cement (OPC) were selected as composite curing agents, while anionic polyacrylamide (APAM) and slaked lime were used as a mixed flocculant. Laboratory experiments were conducted to examine the effects of different dosages of curing agents and flocculants on deposition dehydration, strength characteristics, water content after curing, as well as the spatial distribution of them under the combined method. Additionally, the conventional sludge solidified method treated by GGBS and OPC (denoted as the GCSM) was also investigated and compared. The results indicate that increasing the dosage of curing agent from 4.5% to 10.5% enhances the shear strength of samples treated with VP-FSCM by up to 3–5 times compared to those treated with GCSM. The optimal ratio for the composite curing agent is GGBS/OPC = 1, with optimum dosages for the composite flocculant composed of APAM at 0.125% and slaked lime at 1.5%. When admixture dosage is optimal, it allows for better utilization of the advantages from coupling effects such as flocculation dehydration, vacuum preloading, and chemical curing, thereby significantly improving mechanical properties of the sludge.