Friction stir processing (FSP) was adopted to carry out the preweld surface treatment of diffusion welding on 6063 aluminium alloy. The experimental results reveals that after FSP, a fine-grained layer formed on the surface, resulting in a clear gradient microstructure. The cross-section is divided into three areas: the stirring zone, the thermal mechanical influence zone and the base material zone. To evaluate the optimization effect of surface treatment methods, the influences of different surface treatment methods (no FSP, unilateral FSP, and bilateral FSP) on the microstructure and mechanical properties of diffusion-welded joints were compared. Without FSP treatment, many unwelded defects are significantly formed at the interface. In contrast, the interface healing effect after the unilateral FSP treatment improves to a certain extent compared with that without the FSP treatment. The interface holes are flat, and the grain sizes on both sides of the joint are quite different. When the samples are subjected to bilateral FSP, the interfacial bonding effect is greatly improved. Except for a few granular holes, the remaining holes are welded together. The holes are formed by the shedding of Mg2Si oxide particles at the interface after corrosion. Furthermore, for joints subjected to bilateral FSP surface pretreatment, a study on the influence of holding time on the mechanical properties of the joints is carried out under the process parameters of 570 ℃-4 MPa. The results show that when the holding time is extended from 30 min to 90 min, although the yield strength changes little, the tensile strength increases significantly, reaching up to 111.8 MPa (85.3% of the strength of the base material), and at the same time, the elongation also increases with increasing time, reaching 20.1%. The analysis of the fracture morphology simultaneously reveals the transformation of the fracture mode. With increasing holding time, the number of dimples increases, indicating that the fracture mode gradually evolves from brittle fracture to ductile fracture.