Abstract: The safe and efficient immobilization of high-level liquid waste(HLLW) is a critical global challenge. Vitrification utilizing a continuous Joule-heated ceramic melter(JHCM) is currently the most effective technology for HLLW treatment. During operation, HLLW and glass-forming additives form a floating layer of unmelted batch materials, known as the cold cap, on the melt pool. The cold cap coverage rate significantly governs heat transfer dynamics, physical field distributions, and operational safety. Due to the extreme high-temperature and high-radiation environment inside the melter, directly obtaining internal physical field data is exceedingly difficult. Therefore, establishing an accurate predictive model for the melter’s performance is crucial for ensuring safe operation. In this study, a comprehensive three-dimensional numerical simulation was conducted using COMSOL Multiphysics software to systematically investigate the impact of different cold cap coverage rates(25%, 50%, 75%, and 90%) on the JHCM’s melting efficiency and safety. The computational model coupled fluid dynamics, heat transfer, electric fields, and Joule heat, utilizing approximately 200 000 mesh elements. The molten glass flow was treated as an incompressible Newtonian fluid in a laminar regime, whereas the plenum gas flow was simulated using a turbulent model. The simulation results demonstrate that as the cold cap coverage increases from 25% to 90%, the average temperature within the melt pool rises from 1127 ℃ to 1241 ℃, an increase of approximately 10%. Conversely, the expanded cold cap effectively insulates the upper space, causing the average temperature in the plenum to drop drastically from 897 ℃ to 635 ℃(a 30% reduction), and the exhaust gas pipeline temperature to decrease from 746 ℃ to 501 ℃. These variations profoundly influence electrical and thermal distributions. Under a low coverage mode, the distributions of current density and Joule heat within the molten glass are notably heterogeneous. Excessive local current density concentrates near the electrodes(exceeding 5000 A/m²), significantly exacerbating the risk of electrode ablation. In contrast, under high coverage conditions, the Joule heat distribution homogenizes remarkably, with regions exceeding 80 kW/m³ increasing to 54% of the pool volume. Energy distribution analysis shows that at 90% coverage, thermal energy dissipated into the plenum diminishes to 16.0 kW, compared to 73.5 kW at 25% coverage. Accordingly, heat flux from the melt pool to the cold cap escalates from 11.5 to 22.8 kW/m². Consequently, the comprehensive melting efficiency of the melter undergoes a near-twofold enhancement, surging from 275.4 to 545.3 kg/(m²•d). In conclusion, maintaining high cold cap coverage optimizes internal multiphysics fields, mitigates electrode burnout risks, and maximizes vitrification efficiency.