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Various deterministic and uncertain factors are widely present in mechanical equipment and their working conditions. Attenuation of the contact stiffness of bolted joints caused by bolt loosening will directly affect the dynamic characteristics of the joint surface and the working state of the equipment. This paper proposes a contact stiffness attenuation model that considers time-varying uncertainty of the bolt pre-tightening force as a theoretical basis for designing the initial bolt pre-tightening force and secondary tightening strategies. First, the microscopic contact mechanism of the bolted joint surface was revealed, and then a joint contact load and contact stiffness model based on Hertz contact theory and fractal theory was established. A nonlinear mechanical model of the bolted contact surface was obtained. Considering the decrease in bolt pre-tightening force during service, a contact stiffness attenuation law for bolted joints is explored, and a model of the connection reliability of group bolt joints with time-varying uncertainty is established. The bolt pre-tightening force of a railway locomotive was taken as an example and the connection reliability of group bolt structures was predicted, and it is defined that the bolted structure fails when the contact stiffness drops to 80% of the initial value, and the reliability of the group bolts will drop sharply on the 15th day after service, and will almost completely fail on the 22nd day. The proposed theory improves the accuracy of contact modeling and provides a new direction for bolt tightening strategies.