In　order　to　implement　automatic　welding　of　non-ferromagnetic　material　in　half-structured　environment,　a　new　type　of　mobile　unit　of　automatic　welding　system　which　can　realize　composite　motion　is　designed.　The　mobile　unit　can　move　quickly　by　Adsorptive　mechanism　of　frame　type　structure　on　the　surface　of　non-ferromagnetic　material,　to　leap　over　obstacles　by　eject　mechanism　on　the　leg,　to　turn　by　the　turning　mechanism　between　outer　frame　and　inner　frame,　to　implement　uniform　motion　of　the　executing　mechanism　by　the　dynamic　vector　synthesizing　of　speeds　of　linear　motor　and　relative　motion　between　outer　frame　and　inner　frame.　The　results　of　simulation　showed　that　the　mobile　unit　can　make　the　executing　mechanism　leap　rapidly,　high-accuracy　linear　motion　and　turn.