The flange transition zone of hydraulic cylinder often uses rounded corner to smoothly connect with the supporting surface of cylinder, and it is easy to produce strong stress concentration at the connection between rounded corner and straight sections, leading to produce fatigue cracks and even partial detachment of the flange, resulting in huge economic losses. Therefore, according to this issue, four transition lines were optimized for a hydraulic cylinder with a nominal force of 6000 kN, including rounded corners, oblique lines with rounded corners, concave lines with rounded corners, double exponential “hyper-ellipse” parametric equations. Then, based on the shape and boundary axis-symmetric characteristics of the hydraulic cylinder, the finite element model was established by using a planar axis symmetric model, and the design variables described by the four transition lines were optimized and calculated by the optimization design module embedded in ANSYS. Furthermore, taking the minimization of the maximum equivalent stress in the flange area as the optimization objective, the design parameters of the four transition lines were obtained. Only the optimization results of the double exponential “hyper-ellipse” parametric equation were worse than the original structure, and the other three transition lines effectively reduced the stress concentration in the flange area of the hydraulic cylinder, which was recommended for engineering applications.