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.