Abstract: Engineering calculation method to the in-plane instability of non-reinforced U-shape bellows under combined internal pressure and axial compression displacement is established through calculating formulas for meridional bending stress es due to pressure σ₄ and pressure σ₆ at different positions of bellows based on beam mechanical model. The corresponding criterion of the bellow affected by internal pressure and axial compression displacement is also defined according to experimental data and calculation. The experimental data shows that the engineering calculation method and criterion are effective to engineering application and safety design of bellows.