In order to investigate the flexural behavior of ultra-high performance concrete (UHPC) lightweight composite decks under local wheel load, four demountable steelUHPC composite slabs connected by high-strength bolts were designed and four-point bending test was conducted. The influence of steel plate type and spacing of shear connector on the flexural characteristics of demountable steelUHPC composite slabs was analyzed, including failure mode, load-deflection curve, interface relative slip, crack width, and sectional strain distribution. Results showed that under positive bending moment, the failure mode of composite slabs adopting Q355 steel plate was that the high-strength bolt was cut off. While the failure mode of composite slabs using steel plate with negative Poisson’s ratio (NPR) was as follows: part of high-strength bolts was cut off, part of pre-embedded elongated nuts with cushion was pulled out, and UHPC collapsed due to instantaneous instability. Besides, under the same spacing of high-strength bolts, the relative slip of plate end of composite slabs employing NPR steel plate was relatively small, indicating that NPR steel plate can effectively delay and restrain the relative slip between steel plate and UHPC plate, thus improving the synergistic deformation capacity, flexural stiffness, and flexural bearing capacity for composite slabs. According to the sectional strain distribution analysis, due to the negative Poisson’s ratio effect, high stiffness, and high yield strength of NPR steel plate, the tensile strain between NPR steel plate and the bottom UHPC layer maintained strain compatibility during the whole loading process, and the upward displacement for sectional plastic neutral axis could be ignored with increasing load. Therefore, under the premise that NPR steel plate is employed to improve the flexural performance of steelUHPC composite slab system, the thickness of UHPC should be reasonably matched with the performance of NPR steel plate, so as to give full play to their material properties, and avoid the buckling failure prior to the material strength failure of UHPC.