The embodiment of the invention discloses a bolt fastening robot for transmission line and a control method thereof. Therefore, problems that security of hot-line work of workers is affected because of gradually reduced distances between cross arms and phase spaces due to wide application of a compact overhead line structure and a multi-loop overhead line structure, and the hot-line work can not be carried out timely and efficiently because of severe shortage of professional workers with hot-line maintenance skills and strict requirement of the physical power of workers working high above the ground and thus reliability of power supply is affected can be solved. The bolt fastening robot is composed of an image processing unit, a computing unit and a first mobile unit.