The utility model discloses a knife for machining a highlight decorating side. The knife is used for a triaxial lathe and comprises a knife rod part and a knife edge part. The knife rod part is used for being clamped on a clamp of the triaxial lathe. The knife edge part is arranged on the knife rod part. The included angle between the edge side of the knife edge part and the upper surface of a machining platform of the triaxial lathe is equal to the included angle between a chord line of the decorating side and the upper surface of a machining platform. The knife for machining the highlight decorating side is suitable for machining the triaxial lathe and can machine the highlight decorating side with mirror face effect.