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The demand and requirements of large-size magnesium alloy profiles in the field of rail transit and other fields are increasing. The high strength tough magnesium alloy profiles have high trial mold cost, long development cycle and difficult preparation. In this paper, the HyperXtrude software is used to simulate the profile structure of the large-size high-strength and tough Mg-9Gd-4Y-1Zn-0.8Mn alloy rail transit support beam. Under the premise of ensuring the successful extrusion of large-size high-strength and tough magnesium alloy support beam profiles, the preferred process is determined. The extrusion temperature is 470 ℃, the extrusion speed is 0.3 mm/s, and the maximum pressing force required under the process is determined which is 34606 KN. The large-size high-strength and tough magnesium alloy support beam profile used in rail transit with good surface quality, uniform composition and homogeneous structure is successfully prepared. The tensile strength of each part of the profile is above 370 MPa, the elongation is above 10%, and the highest strength is 391 MPa. The strength of the aging state is obviously improved, reaching 460 MPa or more, and the elongation is slightly decreased, exceeding 8%. The highest strength reaches 475 MPa.