To improve the machining stability and surface quality of jet electrochemical machining (Jet-ECM), and analyze the material removal mechanism in Jet-ECM of copper (Cu), a novel kind of phosphoric acid passivation electrolyte was used in this paper. The novel electrolyte used phosphoric acid as a viscous film-forming agent, ethanol as a wetting agent, benzotriazole (BTA) as a corrosion inhibitor, ammonium acetate and lactic acid as surface oxide removers. The mechanism of viscous film formation was studied through the polarization curve and impedance tests, and the mechanism of ethanol and BTA were studied by cyclic voltammetry curve (CV). The research results showed that with the increase of phosphoric acid concentration, the viscous film could be effectively formed and the surface quality could be improved. Ethanol could improve the hydrophilicity of the electrolyte, which was beneficial for the discharge of products. At the same time, ethanol had a passivation effect that could improve the surface quality. The BTA protected the Cu surface through two ways:physical film and Cu(I)-BTA polymer film. The new acid electrolyte was used for Jet-ECM of Cu, and the surface roughness of the jet center area was reduced to 14.5 nm and the surface roughness of the stray corrosion area was reduced to 157 nm after machining. Compared with traditional NaNO3 electrolyte and pure phosphoric acid electrolyte, it could effectively avoid precipitation and significantly reduce the surface roughness of the jet center and the stray corrosion area. Acid passivation electrolyte based on phosphoric acid group is adopted, it can improve the processing stability and surface quality of pure copper workpiece machined by electrochemical jet. This study improves the surface quality and stability of stress-free machining methods and has great implications for the further understanding of the electrochemical removal mechanism in Jet-ECM of Cu.