The invention relates to an installation method for a high-altitude rib plate of a widening platform. The method comprises the steps that 1) multiple safe channels are manufactured, each safe channelcomprises a baseplate and fences installed at the two sides of the baseplate, multiple horizontal steps are welded to the baseplate in the length direction of the safe channels, multiple handrails arewelded to the sides, not connected with the baseplate, of the two fences at equal intervals, and a top hook and a bottom hook are connected to the two ends of each safe channel respectively; 2) an automobile crane is adopted for hoisting the safe channels, one top hook is hooked on the upper portion of a triangular rack with an installed conical segment, and one bottom hook is hooked on an external structure of the installed widening platform; 3) by adopting the method in the second step, the other safe channels are evenly installed in the circumferential direction of the conical segment at equal intervals; 4) workers enter the safe channels, hold the fences by hand, climb to the tops of the safe channels, and the high-altitude rib plate of the widening platform is installed. The installation method is high in construction efficiency and convenient to operate, the safety of construction personnel can be effectively guaranteed, and after construction, the requirement of the construction quality of the rib plate is completely met.