Transforming sludge into fluid solidified soil is a novel approach for recycling waste material into treasure and resource utilization. This study aims to evaluate the application performance of an alkali-sulfate activated cementitious curing agent in fluid solidified soil. Using the central composite design （CCD） method， the study elucidates the mechanisms by which water-to-solid ratio and curing agent dosage affect the fluidity， unconfined compressive strength， and water stability coefficient. It also explores the underlying strengthening mechanisms through the evolution of the meso-pore structure. The results indicate that fluidity increases linearly with higher water-to-solid ratios and curing agent dosages. The water stability coefficient negatively correlates with the water-to-solid ratio but positively correlates with the curing agent dosage， although the positive effect diminishes with increasing dosage. In terms of strength development， early-age strength is controlled by the combined effects of water-to-solid ratio and curing agent dosage， while mid- to long-term strength （≥7 d） is primarily dominated by curing agent dosage alone. Lowering the water-to-solid ratio or increasing the curing agent dosage promotes the formation of a dense gel network within the solidified soil， significantly enhancing the unconfined compressive strength. The models established based on CCD show high consistency between predicted and experimental values （R2>0.94）， confirming the reliability of the models. Multi-objective optimization results reveal that when the water-to-solid ratio is 0.87 and the curing agent dosage is 12.1%， the fluid solidified soil not only meets the basic requirements for subgrade engineering （fluidity>80 mm， 28 d unconfined compressive strength>1.0 MPa， water stability coefficient≥0.8）， but also achieves full development of strength across all ages.