The utility mode relates to an ultrahigh power transmission line inspection robot and particularly relates to a power transmission line inspection robot mechanism adapted to a large span. The power transmission line inspection robot mechanism comprises a first guide rail, front and rear arms mounted on the first guide rail, a power supply controlling case which moves back and forth along the first guide rail and which adjusts the center of mass of the inspection robot mechanism. The front arm and the rear arm are identical in structure, and each of the front and rear arms comprises a travelling mechanism which travels on a power transmission line, a rotation joint which drives the travelling mechanism to rotate, and a transformation mechanism. One end of each transformation mechanism is connected on the first guide rail, and the other end of the transformation mechanism is connected with the travelling mechanism through the corresponding rotation joint. Each transformation mechanism comprises a parallelogram-shaped body and a moving mechanism, wherein the moving mechanism is mounted on any diagonal line of the body. The shapes of the parallelogram-shaped bodies are changed by telescoping the moving mechanisms, so that transformation of the front and rear arms and the elevation of the travelling mechanisms are achieved. The power transmission line inspection robot mechanism is advantaged in that the obstacle-crossing span is large, obstacle crossing is stable, time for crossing double hanging points is short, the obstacle-crossing process is simple, power required is small, the application range is wide, and the like.