Reinforced concrete calculation theories at the present stage are mainly based on experimental results of small size specimens, studies on the size effect of flexural capacity of large-scale reinforced concrete beam are not sufficient. Main research contents in this paper are the size influence on bending performance of under-reinforced beam, which can help to revise the related design theory. The paper collects test information of five different section size samples in different loading stage, including bearing capacity, deflection, section strain and so on. The total number of specimens is 13.Based on test failure process, development of cracks and deflections, analysis on section stress and strain, the safety of related bearing capacity formula is verified in research scale.It is also found that the safety reserve of calculation theory shows a trend of decrease with the specimen dimension increasing. As for large-size bending specimens, the code value of ultimate strain under non-uniform compression is bigger than test value.