•Damaged RC slabs strengthened with UHPC layer were modeled and verified by experiments.•Two different load patterns: negative bending moment and positive bending moment.•Existed cracks in RC slab was modeled by geometry discontinuous before strengthening.•Two different interface modeling concepts: adhesion and friction (AASHTO) and friction only (ACI). Ultra-high performance concrete (UHPC) has been developed as an innovative cementitious based material. It can be used for repairing and strengthening existing reinforced concrete (RC) structures because of its excellent mechanical performance, such as high tensile and compressive strengths, long-term durability, and low permeability. However, when using UHPC to strengthen existing RC structures for flexure members, there is limited information on simulating existed cracks in RC structures and considering interface modeling between RC substrate and UHPC overlay. This research developed a finite element (FE) model to investigate flexural behaviors of UHPC-RC composite slab with introducing existed cracks in RC substrate by geometry discontinuous, approximately matched with experimental results previously published by the authors. Meanwhile, based on recent research on the bond strength of UHPC to concrete, a UHPC-RC interfacial model was included in the FE model. The FE model was validated with experimental laboratory results previously published by the authors, and a good agreement was obtained between numerical and experimental results. Finally, a parameter study was conducted to investigate the strengthening effects and optimizing strengthening parameters by using the developed FE model. Results showed that the effect of existing cracks on the ultimate flexure capacity of UHPC-RC cannot be neglected, and the interface model has a precise accuracy in FE modeling.