The invention discloses a connecting device for measuring torque of inner hexagon bolts. By the aid of the connecting device, the problem that existing connecting devices for connecting torque wrenches with inner hexagon bolts are integral connecting pieces, square keys are not fastened with square grooves and the torque measuring accuracy is affected can be solved. A first connecting piece of the connecting device comprises an upper cuboid and a square key, a square groove is formed in the upper end surface of the upper cuboid, two first threaded through holes are formed in a side wall of the upper cuboid and correspond to the square groove, a second connecting piece of the connecting device comprises a lower cuboid and a cylinder, a connecting groove is formed in the upper end surface of the lower cuboid, two second threaded through holes are formed in a side wall of the lower cuboid and correspond to the connecting groove, a circular groove is formed in the lower end surface of the cylinder, a regular hexagon column is arranged at the axis of the circular groove, the first connecting piece and the second connecting piece are arranged in the up-down direction, the square key and the connecting groove are fittingly mounted, the corresponding first bolt is connected into each first threaded through hole in a threaded manner, and the corresponding second bolt is connected into each second threaded through hole in a threaded manner. The connecting device has the advantage that the connecting device is used for measuring the torque of the inner hexagon bolts on steam turbine units.