The modified layer was prepared by adding carbon fiber (Cf) and TiB2 to 7075 aluminum alloy using friction stir processing (FSP). The metallographic microscope was used to analyze the microstructure of the different regions of the modified layer. The results show that the grain refinement of the modified layer is remarkable and there is an obvious structure of “onion ring” in the modified layer. The closer to the stirring core zone, the higher the grain refinement degree will be. In the core area, the large granular carbon fibers are embedded in the substrate, and the fine granular TiB2 is distributed around the carbon fibers. Furthermore, there is a clear boundary between the thermo-mechanically affected zone and the heat affected zone, and the boundary has a certain inclination. In the direction of processing depth, the grain refinement is most obvious in the middle layer, and there are obvious metal particles embedded in it.