Based　on　flow-conservation　law,　a　network　traffic　monitor-node　selection　algorithm　is　proposed　to　effectively　select　the　network　traffic　monitor-node.　The　problem　of　the　monitor-node　selection　is　regarded　as　a　problem　to　find　out　the　weak　vertex　cover　of　a　graph.　This　algorithm　marks　all　of　the　network　nodes　with　the　triple　tags　and　recursively　compares　and　replaces　those　triple　tags.　The　network　traffic　monitor-nodes　are　selected　by　the　final　triple　tags.　Simulations　show　that　the　algorithm　needs　not　to　know　the　whole　network　topology.　It　can　dynamically　eliminate　the　unavailable　network　nodes　to　effectively　solve　the　problem　of　the　monitor-node　selection.