A series of elastoplastic large deflection analyses is performed by FEM in order to clarify the buckling/plastic collapse behaviour of stiffened plates subjected to combined longitudinal thrust and lateral pressure on the plate side. Double span/double bay or triple span/double bay models are employed to consider the plate continuity. It has been found that the continuous stiffened plates have three collapse modes, which are Stiffener-Induced failure caused by stiffener yielding, Plate-Induced failure by local plate collapse and Hinge-Induced failure by the formation of plastic hinges predominantly in bending. Based on the collapse behaviour observed by FEM, a simplified method to estimate the ultimate strength of the stiffened plates under combined thrust and lateral pressure is developed. Through the comparison of the estimated ultimate strength with the FEM results, the validity and usefulness of the proposed method are demonstrated.