<p>In this paper, the plastic deformation of AZ31 magnesium alloy thin-walled tubes during four passes of ball spinning is taken as the research object, and a multi-body kinetic contact model is established and solved based on the contact relationship between the ball and the tube billet; the molding accuracy and equivalent force distribution in the process of four passes of ball spinning are studied, and the microstructural evolution of the tube billet material in different passes of ball spinning is investigated by using the electron backscattering diffraction (EBSD) technique. The results show that after the billet is rotated by the first and second passes, the fluctuation of the surface thickness in the forming area is small, and the forming accuracy is high; after the billet is deformed by the first two passes, there exists a certain amount of internal stress in the interior, and it is more difficult to form the third and fourth passes, and the range of fluctuation of the surface thickness in the forming area is also larger; the stacking height in front of the ball rises with the axial feeding of the ball and decreases by the increase in the number of passes of the maximal stacking height. In the EBSD results, the grains were gradually refined to 1.65 µm with the whole spinning process; the recrystallized grains were increasing in the first to third passes, and some of the recrystallized grains were further deformed in the fourth passes, which led to a slight decrease in the volume fraction of recrystallized grains, and the plastic deformation in the spinning process was mainly coordinated by the prismatic slip and with the increase of the spinning passes; the plastic deformation during the spinning process is mainly coordinated by the prismatic slip, and with the increase of spinning passes, the Schmid factor of prismatic slip increases, and more prismatic slip systems are initiated to coordinate the larger plastic deformation.</p>