Abstract: In order to obtain the true bearing capacity of dangerous and old pre-stressed concrete box girder bridges, experiments on flexural and shear bearing capacity of full-scale dangerous and old pre-stressed concrete small box girders are conducted to study the mechanical degradation behavior of dangerous and old pre-stressed concrete box girders. Through the residual bearing capacity test of the full-scale prefabricated box girder, the load, deflection, strain and crack width of the test girders are measured and analyzed. The residual bending and shear ultimate capacity and stiffness of the small box girders are analyzed. The bending and shear behavior and failure mechanism of precast box girders in danger are obtained. The comparisons between the results of full-scale test and the calculated value of bearing capacity and also the design value of the internal force of I-Class highway are operated to analyze the actual performance of the small box girder made of dilapidated concrete. The damage reduction coefficient is introduced to establish the formula of ultimate bearing capacity of the girder. The test results stated that the structural damage affected the bearing performance of this box girder. The deflection of the test girder in the uncracked stage did not meet the proof stiffness requirements under the variable load of highway bridge. The calculated results of the flexural bearing capacity are basically identified with the result from full scale flexural test, which is 70% higher than the design bending moment by the internal force of the main girder. The results of full-scale shear tests are respectively 32% and 37% higher than the internal shear design values of the girder. The formulas with damage reduction coefficient for calculating the bending and shearing capacity of the concrete box girders is applicable to evaluate the bearing capacity of the dilapidated concrete girders with more accurate results, which can provide a reference for the evaluation and maintenance of massive existing small concrete box girders in China.