Ultra-high performance concrete (UHPC) is often combined with normal concrete in engineering applications due to its high strength. To investigate the flexural performance of reinforced concrete-UHPC composite slabs, a series of four-point bending tests were conducted on four composite slabs, considering the effects of shear span ratio and UHPC layer thickness. The failure modes, bearing capacity, deformation capacity and reinforcement strain of the specimens were analyzed. The test results showed that the concrete and UHPC layers exhibited coordinated deformation at the interface. The artificial rough surface did not fully meet the shear resistance requirements of the composite interface, leading to debonding cracks that reduced the flexural performance of the specimens. Besides, increasing the thickness of the UHPC layer or the shear-span ratio casued the composite slabs more susceptible to bending failure. Increasing the UHPC layer thickness increased the load-bearing capacity of the composite slabs and reduced the rate of post-damage capacity decline, but the improvement was slight. The shear span ratio had little effect on the load-bearing and deformation capacities of the composite slabs.