In order to explore the formation mechanism of nano /ultrafine grains during the power spinning of nano /ultrafine grained cylindrical parts,the body-centered cubic steel 20 with high stacking fault energy was taken as theresearch objective,and the metal flow mechanism during the strong stagger spinning was qualitatively and quantitativelyinvestigated by analyzing the fan-shaped plastic flow field in the section and surface of the deformation area ofsteel 20.Then,the evolution of grain morphology of steel F + P during the severe shear deformation caused by powerspinning was discussed by means of metallographic analysis and TEM.The results show that,like other severeplastic deformation technologies,power spinning results in the stretching of isometric crystals in axial direction withthe increasing thinning ratio of wall thickness,which helps to form cold deformation texture with certain orientationdegree and to further generate tiny isometric ferritic 600 nm grains during the annealing at 580℃ for 1 hour.