It is always a difficult problem to simulate and predict the loosening characteristics of bolted connections under complex systems and loads. In this study, based on an automobile assembly, a dynamic model of rigid-flexible hybrid multi-body system including simplified bolts is established, and the road load spectrum under real working conditions are tested and obtained. The attitude of the model is solved by virtual driven iteration to obtain the lateral relative deformation of bolt connectors. The transverse vibration test of single bolt connection based on bench is carried out, and the attenuation characteristic curve of fastening torque under different transverse displacement loads is obtained, and it is found that it is suitable for Farazdaghi-Harris model function through curve fitting. A transformation method from random load to three-level block load based on pseudo-damage equivalence is established, and the residual value of bolt tightening torque is calculated. At the same time, through comparative analysis, it is found that the loading sequence is insensitive to bolt looseness characteristics. This study provides the engineering feasibility direction for the prediction and optimization of bolt looseness characteristics of system-level machinery under multi-freedom load.