The　urban　sewer　network　is　a　tree-liked-structured　system.　The　junction　design　usually　controls　the　depth　of　downstream　drains　and　thus,　the　quality　of　the　system　design.　At　present,　studies　of　sewer　network　traversing　met　little　success.　Furthermore,　the　global　optimal　design　is　relatively　limited　in　China.　A　new　approach　of　traversing　was　introduced　to　the　optimal　design　of　sewer　network　and　the　optimal　solution　of　junctions　was　specially　researched.　Firstly,　ant　colony　algorithm　was　used　to　design　the　single　drain　for　optimization;　then,　a　new　network　coding　approach　was　adopted　in　sewer　network　traversing.　The　design　process　using　ant　colony　algorithm　was　described　in　several　case　studies.　The　results　showed　that　this　approach　can　make　the　calculation　automatically　and　achieve　the　real　global　optimal　design　of　sewer　network.