A novel C-type flocculation reagent was developed to address the high water content of sludge and the environmental pollution caused by wastewater discharge during the mechanical dewatering of the Zhanjiang Bay undersea tunnel. Microscopic parameters, such as granularity and zeta potential, were analyzed to elucidate the mechanism of flocculation and dewatering in C-type reagents. The applicability of the C-type reagent was further validated in a field test and compared to that of the conventional reagent. The results showed that the C-type reagent reduced the water content of the sludge by approximately 20% and decreased the supernatant turbidity by approximately 79%. The C-type reagent demonstrated enhanced bridging and charge neutralization activities, thereby facilitating the aggregation of fine particles (<10 μm) into larger particles (>50 μm), ultimately enhancing the efficiency of flocculation. The C-type reagent can also facilitate the formation of a calcium drainage channel skeleton with enhanced water permeability and reduce slurry viscosity to improve the efficiency of mechanical dewatering. These effects enhanced the pore size distribution of the slurry, resulting in a range of 346 to 8656 nm, which was approximately 15% to 41% greater than the overall pore size of the original slurry. The on-site utilization of 0.5% C-type reagent, compared to the original reagent, can enhance the solid content of the sludge by 35.7%, resulting in a 20% reduction in cost. Moreover, it meets the required sewage indicators, such as pH, and thus qualifies for direct discharge. The findings of this study offer valuable insights for the selection of environmentally friendly and efficient reagents in the context of the combined flocculation-mechanical dewatering method.