The work aims to obtain the mechanical parameters about the dynamic impact performance of rhombic honeycomb cores with various cell-wall-thickness-to-edge-length (δ/l) ratios, under the in-plane impact loadings with different impact velocities. The finite element analytical model and the simplified energy absorption model were established to evaluate the in-plane mechanical performance of rhombic honeycomb cores. The local collapse mode, V-shaped transitional mode and I-shaped deformation mode were gradually observed with the increasing impact velocities. These three different deformation modes were described and presented in detail. Under various cell-wall-thickness-to-edge-length ratios, when the remaining structural parameters are fixed, the dynamic plateau stress is in linear relation with the square of the impact velocities. At different impact velocities, when the remaining parameters are fixed, the dynamic plateau stress is related to theδ/l ratios with the exponential relationship. Through the physical analysis and discussion of the numerical results, the empirical formula on the relation between the impact velocity and the dynamic plateau stress is proposed.