The　carbon　nanotube　(CNT)　film　is　used　to　fabricate　the　novel　carbon　nanotube　film　cathode　(CNTFC)　gas　sensor,　and　the　working　voltage　of　the　novel　sensor　at　atmospheric　pressure　decreases　to　several　hundred　volts　from　the　value　of　more　than　thousand　volts　of　the　traditional　discharge　gas　sensor,　however　it　is　limited　by　that　the　space　charge　in　the　gap　is　very　hard　to　diffuse　after　the　breakdown　of　the　gap.　This　makes　the　gap　in　conductance　and　next　self-sustaining　dark　discharge　impossible.　To　solve　the　problem,　experimental　study　and　simulation　calculation　were　conducted　to　optimise　the　structure　of　CNTFC.　Five　CNTFCs　with　different　structures　were　designed　and　the　experiments　were　conducted　to　study　discharge　principle.　It　shows　that　the　CNTFC　could　be　chosen　as　a　gas　sensor,　when　it　has　the　three　characteristics　of　low　working　voltage,　working　at　self-sustaining　dark　discharge　state,　and　the　space　charge　in　the　gap　being　easy　to　diffuse.　The　electric　field　distributions　of　the　five　CNTFCs　were　studied　and　calculated　by　using　the　finite　element　method.　The　results　of　the　simulations　and　the　experiments　show　that　it　is　reasonable　to　use　the　CNTFC　with　the　above　three　characteristics　as　a　novel　gas　sensor.　(c)　2006　Elsevier　B.V.　All　rights　reserved.