A two-stage method for dewatering high-water-content dredged slurry by flocculation and vacuum-assisted prefabricated horizontal drain (PHD) was proposed to increase the dewatering efficiency by addressing the issues of serious bending and clogging that are typically encountered when using the prefabricated vertical drain (PVD). Firstly, comparison of model tests using PVD and PHD, respectively, under vacuum preloading indicates that the PHD has advantages of uniform settlement of soil, negligible bending of the drain board, uniform dewatering rate and better dewatering efficacy. For the cases considered in this study, the mass of water drained out by PVD was only 77.4% of that by PHD. Effect of flocculation on the dewatering efficacy was investigated and the results indicated that impact of the flocculant (APAM) dosage on the dewatering rate was significant. With moderate addition of APAM (e.g., 0.45% of dry soil mass), the time required for dewatering was shortened by 35%. Lastly, the influence of sedimentation time (i.e., waiting time before applying vacuum pressure) on dewatering rate was studied. The results showed that if the sedimentation time is insufficient, the effect of flocculation cannot be mobilized fully , and as a result, will lead to significant non-uniform consolidation and reduced dewatering efficacy. The best time to start the vacuum pressure is 24 hours after the beginning of sedimentation.