The invention discloses a method for mounting long-span arch bridge tie bars and relates to the field of long-span bridge construction. The method comprises the steps that two main ropes are arrangedon a first buckle tower and a second buckle tower which are adjacent to each other, one end of either vertical temporary support is arranged on an arch rib, and the other end of either vertical temporary support is temporarily anchored nearby a pier of the first buckle tower. Multiple temporary slings are arranged at intervals and connected into an entirety in series by using connection ropes, thetie bars are connected with pulling cables, the pulling cables are used for pulling the tie bars, and the tie bars are placed on all the temporary slings in sequence. The tie bars are continuously pulled till the two vertical temporary supports can be connected with set portions on the tie bars, and one temporarily-anchored end of either vertical temporary support is disassembled and connected with the corresponding tie bar. The tie bars are continuously pulled till one end of each tie bar penetrates through a pipe, and the tie bars are temporarily anchored. The vertical temporary supports are lifted to the designed elevation and locked. According to the method for mounting the long-span arch bridge tie bars, the required temporary materials are few, the sea channel is not occupied, the working period is saved, and the tie bars are convenient to construct.