Selective laser melting (SLM) technology is widely used in the manufacturing of precision and complex components, but the surface roughness has always been a problem that has plagued the further advances of SLM, especially in the context of the increasing complexity of SLMed parts. Among the surface finishing techniques for additive manufacturing, electrochemical polishing (ECP) has the greatest potential to overcome the accessibility limitations imposed by the complexity of components construction. By combining the latest polishing experiments with the previous research, based on the film theory and the various perspectives, the work aims to introduce a new strategy of over-potential electrochemical polishing (OECP) technology to polish the inner surface of IN738LC fluid channels components fabricated by SLM. During the establishment of the polishing system for OECP, the electrode arrangement with built-in tool cathode was adopted to form a uniform internal electric field. The electrolyte was a mixture of phosphoric acid and sulfuric acid and their volume fractions were experimentally optimized. Besides, the optimal main process parameters for electrochemical polishing of the fluid channel inner surface were determined, including the optimal amount of glycerol added, temperature range, and applied potential. The surface roughness of the fluid channel inner surface was measured by confocal laser scanning microscope and surface profilometer to evaluate the electrochemical polishing effect. The morphological changes of the fluid channel inner surface before and after polishing were compared and analyzed by optical microscopy and scanning electron microscopy. As a result, the best polishing effect was achieved on the IN738LC fluid channel inner surface in a mixed electrolyte of 60 Vol% phosphoric acid and 15 Vol% sulfuric acid, and the surface roughness was reduced the most in this electrolyte. Phosphoric acid and sulfuric acid concentrations together affected the dissolution behaviour of IN738LC as the changing ionic concentration affected the rate of charge transfer and indirectly controlled the current density, which was reflected in the trend changes of the polarization curve. The additive glycerol played an important role in adjusting the electrolyte viscosity. Adding 6mL of glycerol to every 100 mL of mixed electrolyte could improve the polishing effect further, because the formation of the film was greatly affected by the viscosity of the electrolyte and an appropriate electrolyte viscosity also resulted in a moderate thickness of the film. Elevated temperature could increase the activity of the reactants and increase the reaction speed, while excessive temperature made the electrolyte in an unstable state which was not conducive to the uniform ECP effect. The medium and optimal polishing temperatures were determined to be between 50-60 ℃. The highly selective electrochemical polishing of the rough SLMed fluid channel inner surface was achieved by adopting an applied potential above the limit current plateau region, which could preferentially remove the adhesive particles formed in powder bed technology. The initial surface roughness of the fluid channel inner surface was up to ~10 μm and because of the more complex thermal process, it was lowered to 1.12 μm at an applied potential of 2.3 V (vs.MSE) after OECP. The new OECP method has significant advantages in both local selectivity and global uniformity, the pronounced and selective polishing achieved on the inner surface of SLMed fluid channels has demonstrated its excellent utility value on solving the dilemma of rough surfaces on complex AMed components.