Research conclusions: As sectional design is conducted by means of the direct estimation formula regarding the girder height estimation based on beating capacity of normal section，it not only Call，when meeting the requirements for beating capacity of normal section of girder and economical reinforcement，conduct sectional design by using the limit value of span—height ratio which meets the requirement of deflection control，but also can conduct optimum design of some bending members with large sectional dimension with consideration of control of bearing capacity of normal section and deflection．With the common—used reinforcement ratio scope of q [0．6％，1．5％]，the reinforcement ratio based on the ultimate bearing capacity increases with the increase of the load under the condition that girder sectional dimension and material strength remain unchanged．Meanwhile，the mid—span deflection value increases in spite the girder rigidity is improved to some sense．Therefore，upon calculation of maximum crack width，although the increase of load will lead the change of relevant parameters，but the increase of reinforcement ratio can better control the crack width．