Research conclusions ： For large - sized sectional bended members, the optimum design can be made with the method of controlling cross - section bearing capacity and deflection control to avoid the waste caused by blindly increase of the size and amount of reinforcing bars for the cross-section. With using Matlab, the optimum design of rectangular cross section beam with single reinforcing bar can be easily made, and from it the primary values of the section height and ratio of reinforcement can be obtained and the final design result can be obtained after the slight modification is done in consideration of the requirements of beam, weight and its size, module. For the rectangular cross section beam，its common ratio of reinforcement is within 0. 6% and 1. 6%. With using the substitution principle of reinforcing bar and concrete computing method described in this paper, the substitution outcomes can be easily obtained, which meet the checking calculation requirements of section bearing capacity, deflection control and crack width. The calculation example shows this optimum design method can make perfect economic benefits and is worth wide using.