The invention relates to the technical field of lightning protection devices, and discloses an adjustable transmission line parallel gap. The adjustable transmission line parallel gap comprises a low-voltage end portion and a high-voltage end portion; the low-voltage end portion comprises a low-voltage end front-end sleeve and a low-voltage end front-end core rod that are connected in a spliced manner, as well as a low-voltage end rear-end sleeve and a low-voltage end rear-end core rod that are connected in a spliced manner; the high-voltage end portion comprises a high-voltage end sleeve anda high-voltage end core rod that are connected in a spliced manner; one end of the low-voltage end front-end core rod and one end of the low-voltage end rear-end core rod are respectively provided with an elastic limit button; a plurality of limit slots matched with the elastic limit buttons are uniformly formed on a sidewall of the low-voltage end front-end sleeve and a sidewall of a low-voltageend rear-end sleeve at intervals; a plurality of pin holes are uniformly formed on a sidewall of the high-voltage end core rod at intervals; a plurality of locating pin holes matched with the pin holes are uniformly formed on a sidewall of the high-voltage end sleeve at intervals; and the high-voltage end sleeve and the high-voltage end core rod are fixedly connected via a pin. The adjustable transmission line parallel gap provided by the invention has the beneficial effects that the parallel gap can be adjusted, the use is convenient and safe, and the applicable range is wide.