The purpose of this article is to analyze the failure mechanism of bolted joint loosening under transverse vibration. Firstly, a transverse vibration testing machine was designed and fabricated, and the loosening failure tests of bolted joint with various working conditions were completed. Again, the loosening failure law was examined. Thereafter, the loosening process of the bolted joint under transverse vibration was analyzed theoreticaly, and the mechanisms of bolted joint loosening were revealed. In the end, the finite element model of the bolted joint was developed based on the test model, and the stress distribution of the cross-section in the bolt and nut, the normal stress distribution of the bearing surface of the bolted joint, the tilt curve of the bolt and nut, and the rotation curve of the bolt and nut to the loose direction were simulated, which verified the rationality and correctness of the loosening mechanism. Results indicate that the vibration frequency has little effect on the loosening, and the loosening can be slowed down by reducing the vibration amplitude, or increasing the initial clamping force, or reducing the thread pitch. Affected by the thread rise angle, the slippage of the thread pairs under transverse vibration will produce loosening torque, and the bearing surface of the bolt and nut resists loosening. Under transverse vibration, the nut reduces the tilt through the dislocation between the threaded teeth, so that the nut has a stronger ability to resist loosening than the bolt, and the bolt is more likely to loosen than the nut.