When reinforcing bars for concrete beam according to positive and negative moments, generally, the top and bottom longitudinal reinforcement are determined only by the most adverse effects of the positive and negative moments and the influence of compressive reinforcement is ignored in this calculation, because of not knowing the area of reinforcement which may be used as compressive bar. According to the analysis of this paper, when the reinforcement ratio is high, if not considering the beneficial effects of compressive reinforcement, the waste of materials will be caused. This paper presents a reinforcement design method which can reasonably consider the influence of compressive bar by simple comparison of moments, when not knowing the area of compressive reinforcement. This analysis can concluded that the results by the method in the paper can really achieve the purpose of saving material.