To investigate the flexural performance of damaged hollow slab girders strengthened with UHPC， two hollow slab girders were designed and fabricated to perform the field full-scale experiments， and the failure modes and load-mid span deflection behavior of the unreinforced/reinforced girder were analyzed based on the comparison of field testing results. The experiment results show that the UHPC layer can effectively improve the failure mode of the damaged hollow slab girder， and the reinforced girder exhibits good flexural and deflection capacities. Compared with the unreinforced girder， the cracking and ultimate loads of the reinforced girder are increased by 11.5% and 23.8%， respectively. Moreover， the reinforcement mechanism of the UHPC layer was discussed， and the theoretical formulas were derived to determine the ultimate bending moment of the reinforced girder， and the theoretical values were compared with the experimental results. The results show that the error between the calculated values and experimental ones is only 1.1%， which verifies the accuracy of the theoretical formulas.