Abstract: Based on ultrasonic technology, through the tensile tests of bolts, the differences of bolt axial force-elongation characteristic curves under different external loads were studied, and the influences of different thread frictions on bolt axial force-elongation characteristic curves were analysed. The results show that the yield points of bolts under uniaxial tension and torsional tension are different due to different load directions. In the process of bolt pre tightening calibration, the thread friction has a great influence on the bolt axial force, which will affect the bolt clamping force of subsequent subsystem. Different thread friction states lead to different axial strain rates during tightening and different characteristic curves. The greater the thread friction coefficient is, the greater the torsional stress is and the lower the yield point is. If the axial tensile speeds are the same, the slopes of characteristic curves are the same, whether it is the normal thread friction between coatings or the use of oil to reduce friction. The friction between metals without coating will lead to the plastic deformation of thread tip, and the slope of the elastic section of the characteristic curve is quite different.