In order to study the flexural load capacity of prestressed concrete bridges under the control of nominal ultimate strain of steel reinforcement, relevant formulas are derived according to the assumption of flat cross-section, and the calculation results are compared with those of the current specification calculations. Relying on the reinforcement and reconstruction project of national and provincial trunk highway bridges in Yunnan Province, a single girder static load test was carried out on 2 pieces of 20m prestressed concrete T-girder bridges replaced, to analyze the damage process, damage morphology, and the change rule of mechanical properties at different loading stages, and to compare the test results with the theoretical calculations. At the same time, for the stacked members of the cast-in-place layer and precast T-beam, the calculation method of flexural capacity considering interlayer shear is proposed, and the generalized diagram of simply supported T-beam with standard span of 20m and 30m is taken as a benchmark, so as to derive the coefficient of capacity improvement under different wing plate widths and prestressing bundles. The results show that: the bending capacity of prestressed concrete bridge controlled by the nominal ultimate strain of reinforcement is not much different from the standard calculation results, but when the reinforcement reaches the nominal ultimate strain, the concrete in the compression zone is far from reaching the ultimate strain; after the cracking of the prestressed concrete T-beam, its cracking performance degradation is obvious, but the test value of the bending capacity is still greater than the theoretical calculation value; the cast-in layer has a certain degree of improvement on the bending capacity of prestressed concrete T-beam bridge; the cast-in layer has a certain degree of improvement on the bending capacity of the prestressed concrete T-beam bridge. The cast-in layer has a certain contribution to the flexural load capacity, although the degree of enhancement is small, the interlayer shear force between the cast-in layer and the precast girder is provided by the compressive stress of the cast-in layer, so the cast-in layer should be guaranteed to have high strength and adhesive properties in the structural design.