Based on the limitations of severe plastic deformation (SPD) technology, such as a small effective deformation area and huge forming load, in the preparation of ultrafine grained/nanocrystalline materials, preparing industrial grade bulk ultrafine grained materials is difficult. In this work, a new SPD method, titled 3D-SPD, for preparing bulk ultrafine grained bars is proposed based on the cross-rolling principle. A severe torsion and compression deformation region was constructed using specially-curved conical rolls and guide plates, super large feed angles, and diameter reduction ratios. During the 3D-SPD process, a billet entered the deformation region from the large diameter side of the roll, where the deformation pressure was MPa grade and able to realize the SPD process with effective strains greater than 6.5. A crack control model based on Oyane criteria was established. Through the optimization analysis of damage factors under different deformation conditions, the crack induced by the Mannesmann effect was effectively restrained. Based on theoretical and experimental results, optimal parameters were determined as follows: cone angle 5°, feed angle 24°, diameter reduction ratio 50%, temperature 700^oC, ovality coefficient 1.02, and roll speed 40 r/min. A 25 mm-diameter ultrafine grained bar of 45 steel was obtained by the single pass deformation. The average grain size was refined from 46 μm to 1 μm, and the yield and tensile strengths were increased by 46% and 42%, respectively.