A typical open-die forging process, square bar drawing, is investigated, which is the most commonly adopted process in a programmed open-die forging system that includes press and co-ordinated forging manipulators. The simulation algorithm for the square bar drawing between simple dies is proposed based on the spread coefficient theory and geometric transformation. The geometry of the as-forged workpiece is modelled by using the concept of the Characteristic Point Set; the spatial motion of the workpiece is computed by using the norma-lized co-ordinate transformation; and the plastic deformation of the deforming part is described by using a geometric transformation based on spread coefficient and reasonable simplifications. The real-time simulation algorithm of the programmed forging process is then formed, which can be used in pass schedule design, providing codes for the coordination control of the press and the manipulator, and can be incorporated into the virtual reality integrated forging plant as the process simulation model.