The invention relates to a tool and method for combined machining of an elongated hole by electric spark perforation and reaming. The tool and method are characterized in that the tool comprises a hollow reaming drill, a hollowed-out ferrule, a tool electrode, a conductive gasket and a wire; when the tool is adopted for machining the elongated hole, the tool electrode and a workpiece are connectedwith a positive electrode and a negative electrode of a pulse power supply respectively; as the front end of the tool electrode is continuously fed to the workpiece, a prefabricated hole with a certain depth is formed in the workpiece, and positioning and guiding are provided for subsequent reaming machining. When the hole depth reaches the position of reaming of a hollow reaming drill bit, the hollow reaming drill bit conducts reaming machining on the basis of the prefabricated hole machined by the electric spark perforation; as the tool electrode is continuously fed, the electric spark perforation and mechanical reaming are conducted simultaneously, and machining of the elongated hole is achieved correspondingly. The tool and method for combined machining of the elongated hole by the electric spark perforation and reaming have the advantages of high machining precision, good surface quality, a simple structure, high practicability and the like.