As to zone A, the velocity differences is naglectable. Meanwhile, the velocity differences increase with the strain rate sensitive exponents of viscous medium. The maximum velocity difference reaches to 24.5% in zone C when m is selected as 0.25. In this case, viscous medium moves more rapidly than metal blank. So material of metal blank in zone C is stretched along meridian direction. It can be used to explain the excessive wall thickness thinning in lateral expansion zone. Velocity differen cesdecrease evidently for the conditions of m = 0.15 and m = 0.05, inwhich cases metal blank and viscous medium deform almost synchronously. Thus the tensile stress subjected by ring-shaped blank along meridian direction reduces sharply as a result. And the wall thickness distribution of formed parts tends to be uniform.