Abstract: As a substrate material for power electronic and radio-frequency (RF) devices, the surface quality of 4H-SiC is critical to device performance. To achieve high-quality and efficient polishing of 4H-SiC, this study systematically investigates the role of electrolyte concentration in surface morphology evolution and oxide formation from both thermodynamic and kinetic perspectives, in conjunction with the interfacial impedance characteristics between the 4H-SiC surface and the electrolyte. The study emphasizes the role of electrolyte concentration during the electrochemical mechanical polishing (ECMP) process. Experimental results demonstrate that when using 0.1 mol/L K₂SO₄ electrolyte, both the surface oxide film growth rate and the interfacial impedance effect reach optimal levels. Under the optimized parameter conditions, ECMP achieves an ultra-smooth surface with S_a of 0.343 nm and a material removal rate of 2.109 μm/h. These findings provide important theoretical guidance for optimizing ECMP process parameters and for the rational selection of electrolyte concentration.