The rapid dewatering of a large amount of waste slurry with high clay content produced during slurry shield tunneling in silty clay formation is highly important. To achieve this, flocculation and sedimentation test is conducted on the slurry mixed with polyacrylamide (PAM) organic flocculant to study the variations in the sedimentation rate of the slurry particles. The settlement effect differences of the flocculant are evaluated by analyzing the variation in particle size, supernatant turbidity, and Zeta potential, while the dewatering performance is studied through specific resistance test. The results demonstrate the following: (1) To reach good flocculation and sedimentation effects, the added amount of cationic PAM (CPAM) is 0.12%~0.15% and that of anionic PAM (APAM) is 0.06%~0.09%; the moisture content of the slurry could be reduced by 10% within 2 h. (2) Nonionic PAM does not effectively reduce the water content of the slurry. (3) PAM flocculants agglomerate the clay particles in slurry to form largesize flocs, which is a key factor in realizing the rapid flocculation and settlement of the slurry. APAM has little effect on the Zeta potential of the slurry, whereas an increase in CPAM gradually decreases the Zeta potential. (4) The PAM flocculant can accelerate the early drainage rate of the slurry and reduce the specific resistance of the slurry to a magnitude of 1013 cm/g.