Objective To study the mechanical behaviors of square honeycombs cores under the in- plane dynamic impact loadings. Methods Corresponding finite element simulation and analysis methodology based on cell array was established using the ANSYS / LSDYNA tool. Results With the increasing impact velocities, different deformation modes were observed. The force-displacement curves included four regimes with distinct characteristics. Different deformation modes were observed for a specimen with given configuration parameters at different impact velocities. The dynamic plateau stresses were calculated for the square honeycombs cores with different t / l ratios at different in- plane impact velocities. Conclusion The dynamic plateau stress was the sum of the static plateau stress and the dynamic enhancement due to the inertia effect. The static plateau stress was proportional to the relative density of square honeycombs cores. The dynamic enhancement stress was proportional to the square of impact velocity and the relation coefficient depended on the t / l ratios. The empirical formula of dynamic plateau stress in terms of t / l ratio and impact velocity was given.