A box-core sandwich panel is consisted of top and bottom facing plates and a core of box-shaped stiffeners
connected to facing plates. According to the assumption that shear forces at the mid-height of box-core stiffeners are
equal, a typical segment is studied, and the relative displacement at two ends of the typical segment is obtained by using
unit load method. Therefore, a theoretical equation for calculating the deflection at the mid-span of a box-core sandwich
beam is presented. To verify the accuracy of the equation, finite element software ABAQUS is adopted to carry out a parametric
study to investigate the validity range. By comparing the theoretical and numerical results, the accuracy of the
equation is verified.