Abstract: To elucidate the synergistic effects of physicochemical structural characteristics and the changes in black water deposition behavior caused by black water solid mass fractions on the vacuum dewatering performance of coal gasification fine slag (CGFS), 2 representative fine slags (samples JYHG and YK2) from Yulin, Shaanxi, were selected for systematic dewatering experiments within the solid mass fraction range of 6%−15%. Multiscale characterization techniques were used to analyze the coupled effects of particle gradation, pore structure, and filter cake crack evolution on dewatering behavior. Additionally, energy consumption analysis and life cycle assessment (LCA) were conducted to evaluate the economic and environmental performance under different solid mass fractions conditions. The results show that JYHG, characterized by coarser particles and a complex pore structure (fractal dimension D₂ is 2.838), tends to form loose filter cakes with severe intraparticle water retention. YK2, with a higher proportion of fine particles and greater packing density, exhibits better dewatering performance at low solid mass fractions. However, at high solid mass fractions (≥15%), the filter cake undergoes macroscopic cracking due to the dramatic increase in capillary shrinkage stress. These cracks lead to severe air penetration, causing the vacuum driving force to fail and resulting in an increase in water content. A significant positive correlation was observed between the crack ratio and water content, with crack evolution being the dominant factor leading to dewatering deterioration at high solid mass fractions. Energy consumption and LCA analyses indicate that increasing the black water solid mass fractions significantly reduces the energy consumption per unit mass of dry CGFS and greenhouse gas emissions. However, excessively high solid mass fractions exacerbate the energy consumption per unit water treated and the human health toxicity burden. Considering dewatering efficiency, filter cake structural stability, energy consumption, and environmental impacts, a black water solid mass fraction of 10%−12% is identified as the optimal range for dewatering, which reduces the greenhouse effect while preventing excessive accumulation of health toxicity and landfill risks under high solid mass fraction conditions. This study clarifies the coupled “structure-energy-environment” mechanism governing CGFS dewatering and provides theoretical guidance for low-carbon and efficient disposal of coal chemical solid waste.