Multi-circuit (extra) high-voltage transmission line places important role in China Power Grid. To ensure its running safety, it is better using inspection robots than helicopters or manual inspections. This paper first presents the mechanism of a kind of inspection robot for 220kv double-circuit transmission line, which has 7 degrees of freedom, designed into double anti-symmetrical retractable arms, interactive sliding and suspending structure. Then build up its kinematics equation using D-H method. In order to solve its reverse solution, apply Back Propagation (BP) neural network in MATLAB. The results have shown the method is effective. To verify the mechanism whether is feasible, build up the model in CATIA, translate it into ADAMS to be simulated. The tests of its prototype in lab shows the simulation is correct. Pictures of the robot overcoming the obstacle on the horizontal line and on the jumpers are shown in this paper.