Abstract: Additive friction stir deposition (AFSD) is a novel structural repair and manufacturing technology has become a research hotspot at home and abroad in the past five years. In this work, the microstructural evolution and mechanical performance of the Al–Mg–Si alloy plate repaired by the preheating-assisted AFSD process were investigated. To evaluate the tool rotation speed and substrate preheating for repair quality, the AFSD technique was used to additively repair 5 mm depth blind holes on 6061 aluminum alloy substrates. The results showed that preheat-assisted AFSD repair significantly improved joint bonding and joint strength compared to the control non-preheat substrate condition. Moreover, increasing rotation speed was also beneficial to improve the metallurgical bonding of the interface and avoid volume defects. Under preheating conditions, the UTS and elongation were positively correlated with rotation speed. Under the process parameters of preheated substrate and tool rotation speed of 1000 r/min, defect-free specimens could be obtained accompanied by tensile fracture occurring in the substrate rather than the repaired zone. The UTS and elongation reached the maximum values of 164.2 MPa and 13.4%, which are equivalent to 99.4% and 140% of the heated substrate, respectively.