Abstract: Double nut is a common anti-loosening form of bolt connection in engineering practice. In order to study the mechanical properties of double nut bolts during tightening process, the mechanical model of double-nut bolted joint considering the influence of thread flexibility is established, and the thread forces are analyzed. The calculation method is deduced for the bolt rod axial force and the thread surface contact stress. Analyzed on thread contact state are the distribution law of thread contact stress and, the influence of upper and lower nut tightening torque ratio. The relationship between bolt preload and nut tightening torque is established, and the influence of bolt specification and the ratio of upper and lower nut thickness on bolt axial force distribution and on contact stress are compared. The refined finite element models of double-nut bolted joint are established, and the simulation results are compared with the theoretical results. The results show that the calculation method of the bolt rod axial force presented are consistent with the finite element results very well.