Abstract: The new and old codes for design of concrete structure were compared in this paper,the results showed that the accounting assumption x 2a'swas not available for the new HRB500 bar in the design of double reinforcement rectangle section RC beam,the compressive reinforcement can not reach design value,so a fixed assumption was raised to fit for HRB500 bar. Meanwhile,b was supplemented under correlative formulas and terms of assumption of plane section. When the aboved assumption and b were applied in the design of double reinforcement rectangle section RC beam,the results and analysis proved that the fixed assumption was more adaptive and created less total steel consumption.