Abstract: This paper established the cost objective function for the unit volume of concrete and unit volume of reinforced steel. Then，combined with the computation formula and assumptions of the double-reinforced cross-section beam，the calculation formula of the double-reinforced cross-section beam was deduced and evolved. Finally，the index of the excellent economic reinforcing rate of the double-reinforced cross-section beam was obtained. On the base of that，the two layers of the pure basement（16 palace diagram was used）were modeled and simulated by PKPM software，and the different section sizes of the basement roof beams of the model were tested. The statistical analysis of the trial calculation results verifies the correctness of the excellent economic reinforcement ratio index of the double-reinforced cross-section beam. The proposal of this idea provides a reference for structure designers to design economical beam sections，provides a basis for real estate development enterprises to evaluate the design results of design companies，and opens up a new direction for structural optimization work.

Key words: PKPM, basement roof beams, economic steel-concrete ratio, optimal design, comprehensive cost