In this study, the internal microstructure of the forewings of Allomyrina dichotoma is investigated by scanning electron microscopy (SEM) analysis. The results of SEM test show that the inner microstructure of the forewings possesses an integrated sandwich-like plate supported by trabeculae, which is composed of upper and lower skins of unequal thicknesses, and a honeycomb core with trabeculae. Beetle forewing is a natural composite material composed of chitin fibres and proteins. Also, based on the micro dimensions of the forewings observed by SEM, two groups of micro finite element (FE) models of the forewings (i.e., core with trabeculae and core without trabeculae) are established to compare and comprehensively understand the effect of trabeculae on the mechanical properties of the forewings. The FE simulation results demonstrate that the trabeculae could effectively (1) improve the stress state on the upper skin, lower skin, and core layer of the forewings, (2) improve the overall bending stiffness of the forewings, (3) enhance the peeling resistance between the skins and core layer, and (4) improve the buckling strength of the thin-walled core layer. The unique forewing structure of the Allomyrina dichotoma can provide an excellent bionic model for optimizing the traditional honeycomb panel structure.