Abstract: Aiming at the limited bearing capacity of normal concrete (NC) beams and the superior mechanical properties of ultra-high-performance concrete (UHPC), a UHPC-NC composite beam was proposed, which combined UHPC and NC along the section height. Through four-point flexural test of UHPC-NC composite beams, the effects of different section forms and UHPC heights on the flexural properties of composite beams such as bearing capacity and ductility were investigated. The failure modes and working mechanism of UHPC-NC composite beams were clarified. The reasonable UHPC boundary height of composite beams was obtained, and the ultimate moment calculation model of the composite beams was established. The research results show that there are obvious differences in failure modes between UN-B and UN-T composite beams. Compared to NC beams, the crack load, the yield load and, the ultimate load of UN-B composite beams are significantly increased, and there are positively correlated with the height of UHPC. UN-T composite beams make full use of the toughness of UHPC and show better ductility and deformation capabilities than those of UHPC beams. Based on the plane section assumption and the stress pattern equivalent method, the ultimate moment calculation models of UN-B and UN-T composite beams are established, which can accurately calculate the bending moment of the composite beams, and the error between the test value and the calculated one is less than 10%.