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It is difficult to control technological parameter of magnesium alloy in extrusion. The reasonable matching between extrusion temperature and speed is crucial to the success of extrusion forming. For magnesium alloy AZ31B sheet with width 700 mm and thickness 4 mm, the extrusion process was simulated by Forge software and fracture criterion of Normalized Crockroft & Latham. The results show that, at early stage, the metal located in the upper and lower parts of ingot flows towards the core, meanwhile, the metal located in the left side and right side keeps flowing along the same direction as that of the extrusion speed; at middle and later stage, the metal located in the direction of ±45° flows apart, one part merging with upper and lower metal keeps flowing towards the core, the other part merging with left side and right side metal extends along the width direction of thin sheet. With the increase of extrusion stroke, the length of formed thin sheet increases, and the local high temperature region transfers from both sides to the center section of thin sheet. When the initial extrusion temperature is 400 ℃, the extrusion speed exceeds 1 mm·s-1, and the local high temperature region of thin sheet has a higher temperature. Furthermore, the forming quality and service performance are not easy to be guaranteed. The application of an initial extrusion temperature 380～400 ℃ and an extrusion speed 0.2 mm·s-1 will save the equipment costs significantly, and guarantee the service performance of thin sheet.
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