The invention belongs to the field of power transmission line repair and installation grounding wire operating tools, and particularly relates to a power transmission line grounding wire horizontal installation device. The device comprises a supporting body, a telescopic horizontal rod is installed on the top of the supporting body, a fixing sleeve is arranged at the end of the telescopic horizontal rod and connected with a grounding wire installation jig through a connector, a steel wire rope is arranged corresponding to the grounding wire installation jig, the two ends of the steel wire rope are fastened to the inner sides of the tops of wire clamps respectively, and an annealed copper wire is fixed to the outer side of the wire clamp on one side. An original method that a grounding wire is installed in an artificial wire lead-out method is changed into a method that the grounding wire is installed on a power transmission line tower cross beam remotely and horizontally so that personal safety of operating personnel can be effectively guaranteed, labor intensity is lightened, work efficiency is improved, and the method is easy and convenient to operate; the method is suitable for installation of tension tower grounding wires of various voltage levels.