A method for retrofitting the top slab of the reinforced concrete box girder with high-reinforcement-ratio UHPC is proposed in order to mitigate the loss of the capacity and durability of the bridge due to the cracking of the top slab. Test on the local models of three top slabs of the box girder is then conducted to investigate the effect of this retrofit method on the lateral flexural behavior of the top slabs under the concentrated loads. The test results show that the UHPC layer subjected to tensile stress induced by negative bending moment improves the cracking resistance and stiffness of the retrofitted slab significantly; The cracking resistance of the retrofitted slab is dependent on the elastic tensile strength of the UHPC; Compared with the unretrofitted slab, the load resistance of the retrofitted slab with the crack width of 0.2 mm increases by 255.8%; When the crack width is less than 0.27 mm, a linear relationship between the load and the maximum crack width is found. On the other hand, when the UHPC layer is subjected to compressive stress induced by positive bending moment, the cracking resistance of the slab is determined by tensile strength of the glue used for sealing cracks. Due to premature cracks in the unretrofitted part of the slab, the retrofitted slab subjected to positive bending moment exhibits a little larger deflection, but the growth rates of both the deflection and crack width of the retrofitted slab during the later period are clearly less than those of the unretrofitted slab. In addition, the top slab is well waterproofed by the compact UHPC layer. The retrofitting layer can effectively enhance the stiffness of the tested slab subjected to positive bending moment and control the crack growth. Failure patterns basically meet the expectations, and the theoretical calculation agrees well with the failure loads.