Lithospheric flexure at subduction zones is a major contributor to induce outer rise earthquakes. Here, we modeled the bending deformation of the subducting plate at the southern Mariana using a 3-D plate flexural model. Intraplate stresses were investigated, with the along-strike variable boundary loadings. In order to match the observations of plate geometry, boundary vertical loading near the Challenger Deep has to be set twice of that in other areas. We also compared results between 2-D and 3-D models and found that the difference on estimating plate stress can exceed 20% when there is an along-strike variation in plate bending. Finally, we found that the sharp lateral variation in the σ and the σ corresponded to an outer rise earthquake cluster at the southern Mariana subduction zone, indicating that along-strike variation in σ and σ may be a significant mechanism to cause non-uniform distribution of outer rise earthquakes.