To increase utilization of the old beam bridges with narrow deck and inadequate bearing capacity, the incremental loading balance method (ILBM) was proposed based on theoretical analysis and the widening and reinforcement method with longitudinal and transverse beams. Calculation analysis models of old beam bridges before and after reconstruction were established based on the fundamental principle of elastic mechanics and the principle of generalized force equilibrium, respectively. From the comparison of these two models, the method of calculating and optimizing sectional dimensions of new longitudinal and traverse beams was developed. Finally, the reliability and validity of the proposed method were verified using a 20 m-span simply-supported prestressed concrete girder bridge which is located in Yueyang City, Hunan Province of China. Results show that the ILBM could determine the size of new longitudinal and traverse beams. The stiffness of longitudinal beams is determined by the incremental bearing capacity of the entire bridge, whereas that of traverse beams is determined by the incremental bearing capacity of the single beam. The widths of the longitudinal and traverse beams increase with the increase in their stiffness, and the heights decrease with the increase in bending moment of the old bridge. The study can meet the demand of the reconstruction of old bridges.