The invention discloses a positioning tool, a positioning testing device and a bevel gear circumferential backlash measuring method. The positioning tool is used for simulating rotor centering state and conveniently adjusting the circumferential backlash of a bevel gear. The positioning tool comprises a positioning part and a compressing part, wherein the positioning part is sleeved with a drivinggear and is capable of simulating a rotor, and the positioning part is assembled into an air inlet case together with the driving gear; and the compressing part is arranged coaxial with the positioning part and is in axial propping fitting with the positioning part in order to simulate the centering state of the rotor in working. According to the positioning tool, the rotor is simulated by the positioning tool; the front and the back of the positioning tool are centered through the positioning part and the compressing part; the state that the rotor in actual working is centered by front and back supporting is simulated; the positioning part and the driving gear are assembled into the air inlet case together; the compressing part and the positioning part are in axial propping fitting; an engine is mounted assembled into a rear supporting case, and the air inlet case part is simply assembled, thus the circumferential backlash of a central transmission bevel gear can be measured, and asa result, an engine assembling structure is greatly simplified.