This　paper　will　first　sum　up　the　typical　attacks　and　countermeasures　in　the　network　layer　of　Wireless　Sensor　Network　(WSN),　then　it　will　classify　the　existing　secure　routing　protocols　according　to　the　core　secure　schemes　used　by　them,　and　emphatically　introduce　and　analyze　the　typical　ones　among　them.　Finally　this　paper　will　propose　some　problems　on　secure　routing,　which　requires　further　studying.