This　research　develops　a　portable　all-position　welding　robot　for　the　welding　of　intersected　pipes.　A　complete　procedure　is　adopted　to　conduct　the　design.　The　task　and　motion　of　the　robot　are　analyzed.　Based　on　that,　three　representative　types　of　the　robot　are　chosen　in　the　type　synthesis　of　the　mechanism.　Two　new　indices　proposed　to　evaluate　required　properties　of　the　robot,　along　with　the　traditional　dexterity　index,　are　chosen　to　be　the　criteria　in　the　dimension　synthesis　of　the　mechanism.　Through　the　optimization　by　genetic　algorithm,　the　best　robot　in　type　and　dimension　is　determined　after　comparison　on　their　performances.　Finally,　the　prototype　is　developed.　The　welding　experiment　result　shows　that　the　welding　robot　works　stably,　and　the　quality　of　weld　seam　is　acceptable　simultaneously.　It　has　great　practical　value.　©　2011　Springer-Verlag　Berlin　Heidelberg.