A machining method for three-dimensional open flow channel using high-speed arc discharge layered sweep, which arranges an axial line of an installation shaft of a tool electrode with six degrees of freedom to be perpendicular to an axial line of a to-be machined work piece in an arc discharge process, with the tool electrode sweeping in a closed path in a plane or a curved surface perpendicular to the installation shaft, wherein the path of the sweeping satisfies: a space of an enveloped space being formed by the tool electrode moving in the path and being coincident with a removed portion of the work piece does not exceed a preset machined cavity of the flow channel. The present invention is based on high-speed arc discharge of hydrodynamic arc breaking and realized layered milling and surface machining for three-dimensional open flow channels.