Abstract: To minimize high production cost brought by using the unbonded prestressed concrete, it needs to be calculated right size of the section and matching tendons with the aid of optimization theory. Starting from this point, with the concept of balance load, an economical ratio coefficient of life load is unprecedently Put forth by means of extreme value in optimal area.This method directly economizes the size and tendon by using the economical ratio coefficient, which differs from the trial calculation approach honoured in the past.