A stretchable boring head comprises a tool bit shell, a worm gear, a worm, a transmission tool bit and a flay key. The worm is arranged in the tool bit shell, the transmission tool bit is arranged above the worm in the tool bit shell and screwed by the external thread into the screwed hole of the upper part of the worm, a key holding chamber is arranged on one side of the tool bit shell close to the transmission tool bit, one side of the flat key is mounted in the key holding chamber and the other side is connected with a flat key groove on the transmission tool bit, the transmission tool bitcan stretch and slide only up and down along the flat key, the key holding chamber is provided with a locking screw for locking the transmission tool bit which is propped against the flat key, the tool is arranged in a tool holding groove of the transmission tool bit, the tool holding groove is provided with bolts supporting the tool and round nuts screwed on the bolt, and the tool is fixed on the transmission tool bit by a baffle plate and the bolts on the baffle plate. The invention is applicable for processing inner holes of a long and thin workpiece with small outer diameter and large inner diameter, is easy to adjust and convenient to process, and can be widely applied in on-site processing complex inner holes of large pipelines, mining machinery, port machinery, ship shaft and rudder systems and nuclear power facilities.