Rotating back extrusion is a new forming method which combines rotational motion with common reverse extrusion. It can realize a severe plastic deformation of Mg-13Gd-4Y-2Zn-0.5Zr magnesium alloy and improve its microstructure and properties. Based on rotating back extrusion, the influence of the strain rate on microstructure and properties during the forming process of magnesium alloy cup are studied by using Gleeble3500 thermal simulator. The results show that in the process of rotating back extrusion of Mg-13Gd-4Y-2Zn-0.5Zr magnesium alloy cup, the microhardness distribution of the magnesium alloy becomes more uniform, the grain size inside the sample gets thinner than the outside, the dynamic recrystallization degree increases gradually along the rotation direction and the grain size gradually decreases. As the strain rate increases from 0 s-1 to 0.05 s-1, the proportion of dynamic recrystallized grains increases. When the strain rate is 0.05 s-1, the sheet LPSO phases begin to break up, which makes the hardness of the material increases and inhibits the generation of dynamically recrystallized grain. With the strain rate keeping on raising, the dislocation density increases, which promotes dynamic recrystallization. During the process of rotating backward extrusion, the fluctuation of the forming load increases with time, and the forming load gradually decreases with the raising of strain rate.