This　paper　proposed　a　new　detection　and　prevention　system　against　DDoS　(Distributed　Denial　of　Service)　attack　in　SDN　(software　defined　network)　architecture,　FL-GUARD　(Floodlightbased　guard　system).　Based　on　characteristics　of　SDN　and　centralized　control,　etc.,　FL-GUARD　applies　dynamic　IP　address　binding　to　solve　the　problem　of　IP　spoofing,　and　uses　3.3.2　CSVM　algorithm　to　detect　attacks,　and　finally　take　advantage　of　the　centralized　control　of　software-defined　network　to　issue　flow　tables　to　block　attacks　at　the　source　port.　The　experiment　results　show　the　effectiveness　of　our　system.　The　modular　design　of　FLGUARD　lays　a　good　foundation　for　the　future　improvement.　©　2017　ACM.