Loosening is one of the causes of bolted joints failure. It is of great significance to understand the bolt loosening process in structure design. The loosening process under transverse cycle load was analyzed. The modeling method for bolt loosening simulation was given to investigate loosening process by analyzing changes of contact states and residue preload. Several factors such as dynamic shearing load position, amplitude and frequency were taken into account. The results show that the residual preload of the bolt rises and declines alternately during the loading cycles under transverse cycle load. Complete thread slip has occurred prior to head bearing surface slip. There is no difference on loosening rate whether the load acting on the upper or lower plate. Vibration amplitude is the main factor affecting loosening rate and the greater the excitation amplitude is, the easier the occurrence of loosening will be. Vibration frequency is proved to have little effect on loosening rate. The results agree with the Junker test results and can be extended to complex load conditions.