The end deformation in milling process caused by the weak stiffness of robot is an important factor affecting the surface quality of parts.On the basis of the typical tool and workpiece finite element model,a finite element model suitable for robotic milling was established by adding equivalent robot end with a beam structure.Then,the equivalent mechanical parameters of the beam structure were obtained through the force hammer experiment on the actual robot.The proposed finite element model was verified by milling 6061 aluminum alloy with a ball end milling cutter.The two different stiffness performance indexes of robot were compared and analyzed through simulation.The results show that the established model can correctly simulate the milling process of the robot,and can be used to optimize the robot milling posture；considering processing efficiency and requirements,the surface normal flexibility coefficient index based on workpiece geometry is more suitable for optimizing the posture of robot milling surfaces.