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The marginal-restraint mandrel-free spinning method can solve the problem of instability when spin-forming thin-walled dome blanks. However, when shear-expansion compound forming is adopted for a large-diameter dome, the sinking phenomenon that occurs during the expansion spinning stage makes it impossible to accurately form the dome. Through finite element simulation, it is found that the smaller the roller fillet radius is, the smaller the feed ratio, and the larger the angle is between passes, the more obvious the sinking phenomenon and the worse the forming accuracy. It is further revealed that the mechanism of the sinking phenomenon is shear deformation in addition to expansion deformation in the expansion spinning stage. To solve the sinking problem, this paper proposes a marginal-restraint mandrel-free spinning method with a large bottom reserve. Research shows that a larger roller fillet radius and more passes can improve the contour accuracy of the dome by using the cylindrical roller structure. Finally, it is verified that the new spinning method can achieve accurate forming of large thin-walled aluminum alloy domes with a diameter of 2250 mm and a forming angle of 60°. [ABSTRACT FROM AUTHOR]