In the press forming of some rectangular workpieces such as floor tiles and automobile panel, in order to make the hydraulic pressure distribute on the workpiece evenly and reduce the dimension of the cylinder, the rectangular cross-section piston has comparative advantages to the ordinary rounded cylinder. Finite Element Analysis (FEA) method is used to simulate the stress and deformation distribution of the rectangular cylinder. It is found that the largest deformation region is on the central of cylinder, the corner of the cylinder has the maximum stress. To decrease the deformation and stress, the convex cross-section shape and rounding corner is used, which effectively reduces the deformation and stress. By analyzing the influence of stress and strain under the conditions of different radius of the corner and different convexity of the cylinder wall, structure of the cylinder is optimized.