Extension device for heavy super-long boring bar and assembling and using method

The invention provides an extension device of a heavy super-long boring bar and an assembling and using method. The extension device comprises an extension boring bar, a through center hole is machined in the center of the extension boring bar, a shaft neck is machined at one end of the extension boring bar, a groove is machined in the outer end face, where the shaft neck is located, of the extension boring bar, a positioning skirt edge is formed on the periphery of the groove, and the positioning skirt edge is located in the center of the extension boring bar. A first key groove is machined in the groove in a crossed mode, and a plurality of bolt through holes are evenly machined in the groove and located in the annular area where the shaft neck is located. A positioning spigot is processed on the outer edge of the end surface of the other end of the lengthened boring rod, and the positioning spigot is matched with the positioning skirt edge; a crossed second key groove is machined in the end face where the