Bonded joints are considered weak points in adhesive structures, and the fatigue-related problems of bonded joints have been the focus of numerous researchers. Single lap composite joint specimens were examined in this paper, and tension-tension fatigue tests were conducted on five groups of these specimens using a hydraulic servo fatigue testing machine. The variability in fatigue life of the test specimens under different fatigue loads was compared and analyzed, along with the effects of different fatigue loads on the failure modes at the bonded interface and the relationship between crack length and the number of cycles. It is found that as fatigue load increases, the proportion of the failure area in the adhesive layer gradually decreases, while the area of tearing in the composite fiber layer bonded to the adhesive layer gradually increases. Additionally, as fatigue load increases, the number of cycles at which damage initiation occurs decreases exponentially. Furthermore, slower crack growth was observed when smaller fatigue load values were applied.