The invention provides a telescopic type clamp-on ammeter for overhead line measurement. A plurality of image pick-up pinholes are formed at an iron core clamp head; collected optical signals are guided to an image pick-up component through an optical fiber so as to form a positioning screen; the positioning screen can be adopted as references for the operation of engineering staff; a three-dimensional guide map indicating the space position relationship of the clamp head and a conductor can be simulated in real time according to the positioning screen; and the relative position change of the clamp head and the conductor can be reflected in the three-dimensional guide map, so as to serve as an indication for the engineering staff. According to the telescopic type clamp-on ammeter for the overhead line measurement of the invention, the iron core clamp head and the facing direction of the jaw of the clamp head can be adjusted sufficiently and freely, so that clamping operation in all directions can be facilitated; and the clamp head can be easily locked after the direction of the clamp head is adjusted, so that a single conductor can be separated from a plurality of bunched conductors so as to be clamped.