The work aims to analyze the formation process and mechanism of high-speed milling sawtooth chip in aerospace aluminum alloy so as to provide a theoretical basis for improving the surface quality of the workpiece and prolonging the service life of the tool. By considering the characteristics of milling thickness variation during high-speed milling of aerospace aluminum alloys, reasonable constitutive model and material fracture criterion were selected to simplify the 3D milling as 2D variable thickness orthogonal cutting thermal coupled finite element model and the formation process of sawtooth chips was simulated to verify the accuracy of finite element model through milling experiments. The milling force, milling temperature and morphology of sawtooth chip were accurately predicted within the cutting speed range of 2~16 m/s. As the cutting speed increased, the thickness of the chip, the height of the continuous portion and the shear band spacing all decreased. On the contrary, the shear angle increased as the cutting speed increased. When the cutting speed was 16 m/s, the saw-toothed chips appeared on the side with the larger chip thickness, and gradually disappeared with the reduction of the chip thickness and then became uniform strip-shaped chips. The sawtooth chip was accurately simulated under the change of cutting thickness. A three-stage sawtooth chip formation model considering the variation of the shear band width is proposed. The adiabatic shear process is analyzed by the changes of stress, strain, strain rate temperature inside and outside of the shear band, and the segmentation intensity ratio parameter is used to quantify the degree of sawtooth chip strain and control the shape of the sawtooth chip.