To expand the application of wood as a building material, a new type of I-shaped wood-steel beam that consisted of laminated veneer lumber and cold-formed thin-walled steel was considered in this paper. The shear performance of nine wood-steel composite beams was tested to evaluate the effects of shear span ratio, web thickness, and flange thickness. Then, the failure pattern and failure mechanism of the composite beams were analyzed. The main affecting factors of shear capacities were also discussed. Furthermore, the calculation formula for bearing capacities of composite beams was established and the calculation results were compared with the experimental results. The experimental results showed that the combined effect of composite beams was excellent. The shear capacity was mainly affected by shear span ratio and web thickness. The calculation formula of the shear capacity was established based on the shear flow theory and the specification for structural steel buildings. The formula was derived from the micro-segment balance method and the reciprocal theorem of shear stress. The calculation results according to the formula were in good agreement with the experimental values.