Abstract: Composite stiffened panels commonly used in aircraft inevitably have manufacturing defects and damages from service, leading to deterioration of structural performance. To investigate the impact of initial defects on the stability and load-bearing capacity of hat-stiffened composite panels, axial compression tests were conducted on three types of hat-stiffened composite panels: without damage, with a hole, and with a notch. These defect types are selected based on typical damage threats identified during aircraft manufacturing and service. The results show that the buckling loads of the three types of panels were nearly identical. The ultimate loads of the panel without damage and the one with a hole are almost the same, while the ultimate load of the notch-defected panel decreased by approximately 30% compared to the other two types of panels. To further analyze the buckling and post-buckling behavior of the panels, finite element simulations based on progressive damage analysis were conducted for all three panel types under axial compression. The predicted buckling loads, ultimate loads, and failure modes are in good agreement with experimental results, where the numerical errors between prediction and test data of the buckling load and the ultimate load are within 5%.