To study the crack propagation, local buckling and vertical shear strength of partially filled narrow steel box-UHPC composite beam in negative moment zone, reverse forward loadings were taken to five composite beams with different UHPC layer thicknesses and infilled HSC heights, and the failure process, load-crack width curve, load-midspan deflection curve, midspan strain distribution, and web stress-strain curves were obtained and analyzed. The results show that compared to the specimens with NC flange, the shear bearing capacity of the specimens with half-UHPC flange and full-UHPC flange has not been significantly increased, only increased by 8.3% and 11.6%. Compared to the unfilled specimens, the shear bearing capacity of the half-infilled and full-infilled specimens has been increased by 61.7% and 87.2%, respectively. After replacing part NC with UHPC of the flange, the specimens can effectively control crack development, and infilled HSC in the steel box can effectively reduce local buckling of the web. The calculation method for the shear bearing capacity of composite beams was established with the subitem superposition method, considering the contribution of flange, steel girder, and infilled HSC to the shear bearing capacity, and it can accurately predict the shear bearing capacity of count beams.