Abstract: According to the need of vision control for robotic welding, this paper propose a mathematic model of real-time control for robotic welding, which is based on the principle of robotic kinematics and data from vision sensor. The mathematic model can revise or adjust three dimension trajectory of robots when there arc deviation between real welding joints and teached path. It is not only for straight line shaped joints to be welded, but also for curve and broken line ones. The accuracy of the mathematic model was analysed in detail. The result of the computer simulation has proved that this model is suitable for tracking weld scam.