In this paper,it's proved that in the quadratic equationsof calculating the ultimate stress increment of unbonded prestressed tendonsboth int cantilever and in single span symmetrical and unsymmetricalbending members,the coefficient a_2 may be given the same value.Basedon flexural experiments of unbonded prestrssed beams,the value is confi-rmed to be 0.76.From this conclusion,an unified quadratic equation forcalculating the ultimate stress increment under any loading and supportingconditions is acquired.The result makes the theory of author more perfectand more practicable.