Abstract: To address the unclear mechanisms of preload relaxation and the evolution of rough interface contact in bolted composite joints under vibration fatigue, an online monitoring system was developed using a strain-ultrasonic measurement method. This system was designed to investigate the multi-scale relaxation mechanisms of bolted composite joints subjected to random loading vibration fatigue, and to explore the effects of surface roughness and bolt preload on vibration-induced relaxation. The long-term relaxation behavior of bolted joints was successfully predicted using a Phenomenological Model, based on 12-hour random vibration fatigue relaxation test data. The results demonstrate that ultrasonic reflections are more effective than strain measurements in characterizing changes in the roughness of the contact interface in bolted composite joints. Under the same preload, the reflected ultrasonic energy increases as the surface roughness of the interface becomes greater. Conversely, for the same surface roughness, higher preload results in weaker reflected ultrasonic energy. The preload relaxation behavior of bolted composite joints is influenced by surface roughness, with joints having a rougher initial surface experiencing faster relaxation under the same preload. After vibration fatigue, the reflected ultrasonic energy at the interface of bolted composite joints with rough surfaces is influenced by both bolt loosening and interfacial wear. For joints with smooth surfaces, however, the increase in reflected ultrasonic energy is primarily driven by interfacial wear behavior at the microscale. The findings of this study enhance the reliability of preload relaxation assessments for bolted composite joints, and propose a novel monitoring method based on changes in ultrasonic energy to characterize the wear behavior of bolted composite joints during early stages of bolt loosening.