Tension reducing is one of the important processes in the production of steel tube, especially for the production of small diameter and thin-walled tube. Tension reducing mill has wide work speed range. A series of finished tube with different diameters and wall thickness were produced from a pierced billet by changing the velocity ratio between the rack. It is important to obtain the relationship of the wall thickness variation and tension for the production of satisfied steel tube. A speed calculation model was established for a suitable tension. The setting of the initial velocity field has great effects on the convergence of iteration process or iterations, so it must strictly satisfy the velocity boundary conditions, and must not break continuity condition and approximately meet the constancy of volume.