Abstract: The interface toughening effect and toughening mechanism of low-density aramid staple fiber (AF) on the carbon fiber reinforced epoxy resin composite (CF/EP)-aluminum honeycomb sandwich structure were studied. A composite sandwich beam was prepared, and AF of 6 mm length was made into a thin layer of flocculent fibers for the toughening of the sandwich beam interface layer. The asymmetric double cantilever beam experiment was used to measure the interface fracture toughness of toughened and un-toughened sandwich beams. Compared with the un-toughened sandwich beam specimens, the average critical energy release rate of the toughened specimens is increased by 91%, and the average critical load is increased by 55%. The addition of the AF toughening layer only increases the quality of the sandwich beam by 0.36%, which shows that the method in this paper has a good toughening effect and efficiency. SEM was used to observe the cross-sectional morphology and characteristics of the sandwich beam interface. The microscopic observation results show that during the expansion of the interface crack, the AF forms a bridging microstructure between the panel and the core, and improves the dissipation energy and critical load of interface crack propagation through fiber pull-out, fiber peeling, fiber breakage, etc. On the other hand, in the surplus area of the resin “round corners” around the honeycomb panel, the AF can also improve the bonding performance of the resin and the honeycomb panel, and prevent the honeycomb panel from being pulled out due to the small contact area with the panel. This paper quantitatively measured the macro-toughening effect of AF on the CF/EP-aluminum honeycomb interface, and explained its micro-toughening mechanism. The related findings can provide guidance for improving the safety and reliability of composite sandwich structures.