A　DNA　electrochemical　sensor　was　prepared　by　assembling　layer-by-layer　films　of　poly　dimethyl　diallyl　ammonium　chloride　and　calf　thymus　double-stranded　DNA　(CT-dsDNA)　on　the　surface　of　a　glassy　carbon　electrode　(GCE).　The　assembling　DNA　was　characterized　by　electrochemical　methods　and　atomic　force　microscope.　Ru(bpy)(3)(2+)　was　used　as　electrochemical　catalyst　to　detect　the　oxidative　damage　of　DNA　in　the　membrane.　The　effects　of　incubation　time　and　concentration　of　Fenton　reagent　on　the　damage　of　DNA　were　studied.　The　result　showed　that　it　could　cause　damage　greatly　when　incubated　in　Fenton　reagent　for　15　min.　The　detectable　concentration　of　Fenton　reagent　was　5.0　x　10(-5)　M　FeSO4/2.0　x　10(-4)　M　H2O2.　The　electrochemical　method　possesses　high-sensitivity　for　detecting　the　damage　of　DNA.