This study investigates the ultimate strength of container ships under the combined action of vertical and horizontal bending moments. The ultimate strength of three container ships with different specifications is calculated using single-span finite element models, and the influence of loading sequence and path on the results is discussed. A revised failure envelope equation for the ultimate strength of ultra-large container ships under biaxial bending moments is proposed. Using finite element software, the stress-strain characteristics of stiffened panels are obtained, both with and without initial stresses. Based on this, the existing load-end shortening curve for stiffened panels is improved. A simplified incremental iterative method for calculating the hull girder's ultimate strength under biaxial bending moments is then proposed. A comparison between the results from the nonlinear finite element method and those from the proposed method demonstrates that the latter achieves high computational accuracy, along with simplicity of operation and ease of programming. The findings of this research can be applied to the calculation and analysis of the ultimate strength of container ship hull girders under biaxial bending moments.