The invention discloses a device and method for testing the amount of looseness of connecting pretightening force of an after-fire high-strength bolt. The device comprises a strain tester, a computer, a base, a bolt axial-force bearing device and a torque loading device. The bolt axial-force bearing device is composed of a lower rigid
cushion plate, an elastic steel sleeve and an upper rigid
cushion plate, wherein the lower rigid
cushion plate, the elastic steel sleeve and the upper rigid cushion plate are sequentially arranged from bottom to top, the bolt to be tested can penetrate through the lower rigid cushion plate, the elastic steel sleeve and the upper rigid cushion plate from bottom to top in sequence, a nut is connected to the end, penetrating through the upper rigid cushion plate, of the bolt to be tested in a threaded mode, and a first resistance strain gage and a second resistance strain gage are symmetrically bonded to the outer wall of the elastic steel sleeve. The method includes the following steps that firstly, the looseness state of the bolt, having suffered from fire, to be tested is marked; secondly, the
torque coefficient of the bolt, having undergone a fire suffering test, to be tested; thirdly, the amount of looseness of connecting pretightening force of the bolt, having undergone the fire suffering test, to be tested is calculated. The device and method are high in testing precision, testing results can provide reference for after-
fire safety performance assessment and reinforcement treatment of a steel structure, waste is avoided, potentially safety hazards are reduced, and the practicability is strong.