The rapid development of additive manufacturing technology (AM) has brought opportunities for the design, preparation, and application of high-performance aluminum alloys and Al matrix composites (AMCs). In this review, the application status and prospects of AM in Al-based alloys/composites were addressed. The effect of process parameters on the microstructure and mechanical properties were analyzed in detail. On this basis, the formation mechanism and control strategy of defects in Al-based alloys/composites during the AM process were reviewed. It was found that the wide semi-solid range, high thermal conductivity, high laser reflectivity of Al, high heating/cooling rate, and the resultant great temperature gradient during AM jointly lead to the formation of defects, e.g., coarse grains, pores, and micro cracks. By introducing nanoparticles to increase the nucleation rate, stabilize the molten pool, and increase the laser absorption rate, the density of defects can be significantly reduced. In the future, a deeper understanding of defect control mechanisms,  composition regulation, and reinforcement distribution in AM technologies is needed, as well as the development of special Al powders for AM. This review provides a reference for the development and application of high-performance Al-based alloys/composites fabricated via AM.