Refill friction stir spot welding (RFSSW) has demonstrated broad application prospects in the field of lightweight, high-strength aluminium alloys because of its advantages of high efficiency and superior quality. However, studies on RFSSW for dissimilar materials have not been sufficiently conducted, and a comprehensive evaluation system for joint quality is still lacking. This study focused on the dissimilar combination of 2B06/7B04 high-strength aluminium alloys. Through systematic RFSSW process experiments, relationships between the welding parameters and joint morphology characteristics, microhardness distribution, and tensile-shear properties were established, ultimately achieving process optimization. The optimal process parameters obtained by fitting the regression equation are as follows: rotational speed, 1 600 r/min; press volume, 1.47 mm; press rate, 60 mm/min; and refill rate, 60 mm/min. The corresponding joint tensile-shear failure load is 5.22 kN. The analysis of tensile-shear fracture morphology reveals that the joints exhibit two fracture modes, plug-type fracture and mixed-mode fracture, whereby the failure load of plug-type fracture was lower than that of mixed-mode fracture.