Abstract: [Objectives]With the continuous development of large-scale and complex ships, the number of finite element model grids for hull structures above class of hold increases dramatically, and many large non-linear mechanical problems such as collision, impact and contact are difficult to solve. In order to solve this problem, a simplified method for deformation of bi-directional stiffened plate structure based on orthotropic equivalent is proposed, which simplifies the modeling of ship structure. [Methods]The orthotropic simplification method for two-way stiffened panels of main ship members is studied. The current well-established simplification method for plane stress of one-way stiffened plates is extended to the wider and more complex plane bending problem of two-way stiffened plates. The ratio of total moment of inertia of stiffened plates to moment of inertia of plates in the orthogonal direction is introduced to reflect the structural orthotropism. By substituting it into the equivalent constitutive equation for plane bending of stiffened plates extended, physical orthotropism can be transformed. On the mechanical level, it not only reflects the deformation resistance of the structure, but also takes into account the influence of the membrane forces generated by shifting the neutral axis. By finite element calculation, the deformation modes of four-sided fixed stiffened plates are classified according to different displacement distributions. The result of stiffened plate is compared with that of traditional method and the error analysis is also carried out. [Results]In ship two-way stiffened plates