The invention relates to an installation arm applicable to a power transmission line tower. The installation arm comprises a first extension arm and a second extension arm which are spliced through avertical adjustment assembly, wherein the end surface of the first end of the first extension arm is a first inclined wall, and the end surface of the first end of the second extension arm is a secondinclined wall matched with the first inclined wall; the vertical adjustment assembly comprises a first conversion plate and a second conversion plate, and the first conversion plate and the second conversion plate are arranged on the first inclined wall and the second inclined wall respectively; the first conversion plate and the second conversion plate are the same in structure and matched witheach other, the first conversion plate and the second conversion plate are provided with a plurality of bolt holes, connecting lines of the bolt holes form a square, and the area of the square is larger than the area of the first inclined wall and the area of the second inclined wall. The installation arm applicable to the power transmission line tower can meet the requirements of different monitoring devices and installation positions, and the connection stability of the installation arm is high.