Abstract: Automatic path-layout of multi-pass welding is one of key technologies for thick plate arc welding robot.Multi-pass path-layout strategy of single-sided V-groove and double-sided K-groove is researched.Multi-pass path-layout model based on graph is developed through feature modeling, welding sequence and layout types and so on.Position and attitude of multi-pass path can be automatically generated.Self-defining layout algorithm of multi-pass path-layout is deduced in detail.Moreover, Self-define layout is used to S-seam multi-pass welding of single-sided V-groove.Multi-pass welding simulation and motion program of an arc welding robot are quickly generated, flexibility and efficiency of teach improving.