In order to improve the problem of easy cracking in the negative moment zone of the composite beam, the UHPC-narrow steel box composite beam structure was proposed, and the shear load capacity calculation method of UHPC-narrow steel box composite beam was obtained based on the ultimate equilibrium method and the sub-stacking method. In order to verify the accuracy of the calculation method, one normal concrete narrow steel box composite beam and three UHPC-narrow steel box composite beam specimens were designed with the thickness of UHPC layer and steel fiber as variables, and the test beams were subjected to mid-span reverse loading test to obtain the shear resistance of the composite beam under negative bending moment. The test results show that compared with the normal concrete narrow steel box composite beam, when the UHPC material is used in the wing slab, the crack distribution in the wing slab was more regular, and with the increase of the thickness of the UHPC slab, the crack distribution gradually shows the characteristic of vertical equidistant distribution, and when the thickness of the wing slab was kept constant and the thickness of the UHPC layer was increased by 50 mm, the yield load increases by 12.5% and the ultimate load increases by 8.3%, and the deflection value corresponding to the ultimate load is reduced by 22%. By analyzing the yield deflection as well as the ultimate deflection of each specimen, it can be seen that the yield deflection tends to increase gradually with the increase of UHPC airfoil thickness, and the proportion of the elastic phase of the combined beam keeps increasing. The comparison of the test data with the theoretical calculation results shows that the calculation results obtained by the sub-stacking method can more effectively reflect the contribution of UHPC layer, ordinary concrete layer, steel box, reinforcement and filled part of concrete to the shear load capacity, and the results are more accurate.