Three Mg-8Zn-xCu (x = 0, 0.5 and 1, mass fraction, %) alloys were prepared by friction stir processing (FSP). Meanwhile, the influence of Cu content on the formability and mechanical properties of FSP Mg-8Zn alloys at high rotation rate was investigated by differential thermal analysis (DTA) and backscattered electron (BSE) etc. It was found that with the increasing Cu content, the eutectic in the Mg-8Zn-xCu alloys was transformed from a low-melting-point binary eutectic (346 °C) to a high-melting-point ternary eutectic (434 °C) during solidification process. The predominant eutectic phase was evolved from the Mg₇Zn₃ to the MgZnCu phase. The elevated eutectic temperature effectively suppressed the liquation (i.e., liquid formation) and liquation-induced cracking during FSP. By a Cu content of 0.5%, most of the secondary phases were dissolved into the Mg matrix via the thermo-mechanical coupling effect of FSP. This resulted in a super high Zn solid solution content of 6.62% (mass fraction) in Mg matrix, approximately 4 times of the room-temperature equilibrium solubility. Compared with the FSP Mg-8Zn alloy, the FSP Mg-8Zn-0.5Cu alloy exhibits a 50 MPa increase in tensile strength, achieving 300 MPa, and the elongation to fracture significantly increases from 13.4% to 30.2%. It is the improvement of microstructural uniformity that leads to a simultaneous enhancement of strength and ductility, supplemented by the synergistic action of high-content Zn solid solution strengthening, grain refinement, and MgZn₂ dynamic precipitation strengthening. As the Cu content was increased to 1%, stripe-like particle clusters formed within the processing zone due to the undissolved secondary phases, leading to a significant degradation in tensile properties.