The invention discloses a detecting tool for detecting position error of a flange bolt hole. The detecting tool comprises a chord distance gauge and a reference circle plug gauge. The chord distance gauge comprises a positioning board on which at least five first positioning holes are distributed. The positions of all first positioning holes satisfy a circumferential position error designing requirement for the flange bolt hole. Each first positioning hole is internally provided with a first lining. A first positioning pin is inserted into each first lining. The reference circle plug gauge comprises an extension rod. Two ends of the extension rod are respectively provided with an end plate. The two end plates are provided with at least one pair of second positioning holes. The positions of each pair of second positioning holes satisfy a radial position error designing requirement of the flange bolt hole. Each second positioning hole is internally provided with a second lining. A second positioning pin is inserted into each second lining. The detecting tool can quickly and accurately detect the position error of the flange bolt holes of a batch of flanges, thereby greatly improving detecting efficiency.