The invention discloses a mounting device for an arch rib of a butterfly arch bridge and an arch rib construction method, and relates to the technical field of arch bridge construction. The butterflyarch bridge comprises a main beam and at least one pair of oppositely arranged arch ribs, each arch rib comprises a plurality of arch rib sections; the mounting device comprises two supports which isoppositely arranged and a plurality of rotating adjusting devices, the bottoms of the supports are rotationally connected with the main beam, and the tops of the supports are detachably fixed with thearch rib sections; and each rotating adjusting device comprises an inhaul cable and a stretching device, one end of the inhaul cable is fixed on the arch ribs, and the other end of the inhaul cable is stretched into the stretching device, the stretching device is rotationally connected with the main beam, and the stretching device is used for adjusting the stretching length of the inhaul cable. According to the mounting device for the arch rib of the butterfly arch bridge, the influence of environmental restriction on arch rib construction can be reduced.