In this study, the corrosion behavior of Cu_XCoCrMoNi (x = 0.3, 0.6, 0.9) high-entropy alloys (HEAs) in 3.5% NaCl solution is systematically investigated. The alloy samples show a strong link between copper content and corrosion resistance. It is noteworthy that an increase in copper content promotes element segregation, resulting in an increase in corrosion current density from 2.138 × 10⁻⁷ μA/cm² to 1.8989 × 10⁻⁶ μA/cm² and a decrease in charge transfer resistance from 182.6 Ω·cm² to 42.34 Ω·cm². In addition, electrochemical experiments demonstrate that lowering the copper content in the alloys reduces the spread and depth of corrosion. All alloys exhibit n-type semiconductor behavior, with donor density increasing from 4.792 × 10²³ cm⁻³ to 5.581 × 10²³ cm⁻³ with increasing copper content. Notably, the passive film is characterized by the presence of Cr₂O₃ and Cu₂O as its main constituents. As the copper content in the HEA increases, higher levels of copper oxides in the passive film inhibit the formation of chromium oxides. This degrades the passive film quality, thereby diminishing the overall corrosion resistance.