Abstract: The formulas for the shear capacity of steel-concrete composite beams either in current specifications or proposed by researchers do not consider the contribution of the shear resistance of the flanges of steel beams. Therefore, the calculation results are relatively high. Based on the reasonable material constitutive relations and ABAQUS, a full-scale finite element model of composite beams considering the stress characteristics of the bolts was established to study the shear properties. The accuracy of the model was verified by a comparison with existing test results in the literature. Through a parametric analysis, the boundary shear span ratio of composite beams was determined, and the redistribution rule of the internal force of the studs of composite beams under shear loads was revealed. The combination rule of the steel beam and the concrete wing plate and the sharing proportion of the shear load were also revealed. Based on the superposition principle, a formula for calculating the shear capacity of I steel-concrete composite beams considering the shear contribution of the concrete slabs, steel girder webs and flanges was presented. The comparison results show that the accuracy of the proposed formula is higher than those suggested by GB 50017−2017 and other researchers.