The work aims to study the instability of double-deck A-type corrugated board which is used to constitute corrugated box in the process of stacking for large-/medium-sized shipment packaging via Ansys. First, by using software Ansys in combination with APDL language, the finite element model of double-deck A-type corrugate board was constructed. Then through analyzing the eigenvalue buckling of such model, the minimum critical buckling load which led to the buckling of the board and the deformation after the board buckled could be calculated. Finally the length and height of the board were changed and repeated eigenvalue buckling analysis was carried out for many times to analyze the relation between board length & height and the critical board buckling load. The critical buckling load of the corrugated board was in direct proportion with its length and was in inverse relation with the square of its height. In conclusion, corrugated box constituted by double-deck A-type corrugated board is more applicable to products of lower height, so as to reduce the possibility of instability. But when the force applied on the double-deck A-type corrugate board exceeds the critical buckling load, the stiffener can be added to the outer surface of the corrugate board to share part of the bearing pressure, thus ensuring the board stress to be under the critical buckling load.