During shield tunneling, the dewatering efficiency of the generated high-water-content waste slurry is typically low, and its reutilization process is complex. To address these challenges, this study proposes a synchronous dewatering-cementation regulation process for preparing flowable fill. In this process, a cementitious material—ordinary Portland cement (OPC)—and a high-molecular-weight flocculant—anionic polyacrylamide (APAM)—are simultaneously introduced during slurry pretreatment. This approach allows coupled control of dewatering and cementation during filtration, enabling direct conversion of waste slurry into a pumpable and solidifiable flowable fill. Additionally, systematic investigations are performed on the effects of OPC and APAM dosages and their addition sequences on specific resistance to filtration (SRF), flowability, rheological behavior, and unconfined compressive strength. The underlying mechanisms are elucidated through low-field nuclear magnetic resonance and particle size analyses. The results reveal the following: (1) The synergistic action of OPC and APAM considerably enhances dewatering efficiency, with SRF stabilizing at the order of 10 10 m/kg. Among the tested methods, the O-A-FT process (adding OPC before APAM, followed by filtration) achieves comparable dewatering performance with a lower flocculant dosage; (2) The cured dewatered product exhibits unconfined compressive strength values ranging 102.0－611.3 kPa, satisfying the requirements for grouting and backfilling in underground engineering; (3) APAM promotes floc formation and accelerates dewatering through adsorption-bridging interactions; however, it reduces slurry flowability, which can be restored to above 200 mm after mechanical remolding, exhibiting typical shear-thinning (pseudoplastic) behavior; (4) Mechanistic analysis reveals that the combined effects of electric double-layer compression by OPC and polymer bridging by APAM produce a stable floc framework, which is crucial for balancing dewatering, cementation, and flowability.