Flying shear is an important equipment used by iron and steel enterprises to cut metal billets, and its performance will directly affect the production efficiency of rolling production lines. There are many structural forms of the flying shear mechanism. In this chapter, the four-link structure is adopted, and the simple 3D modeling of its frame, upper and lower cranks, upper and lower connecting rods, upper and lower rockers and workpiece is carried out. And by assembling and simulating, the shearing force of the workpiece and the motion trajectory of the two shearing edges can be obtained during the shearing process. The flying shears are installed on the rolling line to cut the head and tail of the rolling piece horizontally or cut it to a fixed length. During the movement of the rolling piece, the relative movement of the shearing blade cuts the rolling piece. The schematic diagram of the four-link flying shear mechanism is shown in Figure 1. It consists of upper and lower shearing mechanisms, and the shearing blade is fixed on the connecting rod of the four-bar mechanism. In a practical flying shear mechanism, the driving force is input from the lower crank. A pair of helical gears with the same number of teeth drives the upper crank to move at the same rotational speed, and the mechanism cuts the workpiece once for each revolution of the crank. In order to simplify the structure and facilitate the measurement of shear force, the same magnitude of moment is added to the two cranks in Figure 1 to reduce the modeling of the helical gear.