The invention provides a transmission line lightning arrester device and a mounting method. The transmission line lightning arrester device includes a lightning arrester body, a high voltage end electrode, a low voltage end electrode, a gap adjustment assembly, an insulating base, a counter, a post insulator and an angle iron support. The high voltage end electrode is connected with a transmission conductor overhanging yoke plate. The low voltage end electrode is connected with one end of the lightning arrester body through the gap adjustment assembly. The other end of the lightning arrester body is connected with the angle iron support through the insulating base. The counter is fixed on the angle iron support. The angle iron support is connected with a tower. The post insulator is connected between the lightning arrester body and the angle iron support in an obliquely supporting manner. The device and the mounting method of the invention are suitable for the mounting of AC/DC line lightning arresters of different voltage levels and with different body lengths, and have the characteristics of simple device, easy mounting and high reliability.