Bored pile waste slurry generated during bridge substructure construction exhibits high water content and stable colloidal properties, resulting in difficult solid–liquid separation and severe environmental risks. To improve solid–liquid separation and facilitate efficient disposal, this study systematically investigated the flocculation and dewatering performance of the slurry using three typical flocculants (Anionic polyacrylamide (APAM), cationic polyacrylamide (CPAM), and polymeric aluminum chloride(PAC)) through sedimentation tests, supernatant purification monitoring, particle size analysis, zeta potential measurements, and specific filtration resistance tests. The results show that flocculant type and dosage significantly affect treatment efficiency. Organic flocculants outperform inorganic PAC in both particle aggregation and dewatering improvement. Among them, APAM exhibits a prominent low-dose advantage; at an optimal dosage of 0.2–0.3% (corresponding to a pure reagent dosage of 10–15 mg/L based on wet slurry), it effectively reduces the slurry water content and lowers supernatant suspended solids below the 50 mg/L discharge standard within 80 min. Mechanistically, PAC provides primarily electrostatic neutralization, whereas APAM and CPAM achieve composite flocculation through charge neutralization, adsorption bridging, and sweep capture. Benefiting from efficient low-dose neutralization and long-chain bridging effects, APAM significantly reduces filtration resistance and forms stable flocs. Overall, an APAM dosage of 10–15 mg/L is the optimal scheme for in situ treatment of bored pile waste slurry, providing technical support for the efficient management and controlled disposal of engineering slurry under similar geological conditions.