Abstract: The indexes concerning longitudinal， radial and tangential deformations during stretch reducing process for the characterized steel tube are put forward by means of derivation of the plasticity equation of steel tube stretch reducing.The characteristics of effect by the tension coefficient on stretch reducing deformation are calculated and analyzed.And likely，the characteristics of the critical tension coefficient are quantitatively analyzed.As a result，a conclusion is made，covering the aspects as below.Increasing the tension coefficient is helpful for boosting both longitudinal extension deformation and wall thickness-reducing deformation，but counts against diameter-reducing deformation.As for the above mentioned deformations in three different directions，the wall thickness variation is most obviously affected by the tension coefficient.During the reducing process of the steel tube，size of the critical tension coefficient is just only determined by the steel tube D/S ratio，i.e.，the bigger the D/S is，the bigger the critical tension coefficient will be.The size range of the critical tension coefficient as theoretically calculated is 0.35～0.50，and in any case，the value will be less than 0.50.Examples of actual operational applications have demonstrated that it is practical and effective to determine the tension coefficient in accordance with the steel tube D/S value.