The rectangular concrete-filled steel tube (RCFST) has been widely applied into many engineering fields like buildings and bridges. However, the shear mechanisms and the universal design methods under lateral shear forces still remain to be fully understood. In this paper, the composite action of RCFST columns under lateral shear forces was systematically investigated with improved 3-dimensional finite element simulations. The models adopted the improved material constitutive models and detailed contact interactions between the encased steel tube and core concrete. On the basis of available test data and numerical results of parametric studies, the critical shear span ratio and its variation range were determined. The composite effect and interactions between the encased steel tube and core concrete under different configurational dimensions and loading conditions were investigated. Calculation formulas of the shear stiffness and capacity were proposed based on identified features. The cooperative working mechanism and the constraining effect of steel tube were considered. The proposed calculations were then compared to available design regulations in different countries, and yielded more accurate predictions than the others.