In　this　paper,　first,　SnO2/WO3/MWCNT　composite　thin　film　was　prepared　by　magnetron　sputtering　a　tin　(Sn)　target　and　a　tungsten　(W)　target　in　a　RF　(Radio　Frequency)　reactive　mode,　and　the　corresponding　gas　sensor　was　fabricated.　Next,　the　cross-section　morphology,　phase　structure　and　chemical　composition　of　the　film　were　investigated　by　means　of　FSEM,　XRD　and　XPS,　respectively.　Then,　the　gas-sensing　properties　of　the　gas　sensor,　such　as　the　sensitivity,　the　selectivity　and　the　response　and　recovery,　were　tested.　The　results　of　the　experiments　show　that　the　composite　thin　film　gas　sensor　exhibits　excellent　gas-sensing,　and　that　is　sensitive　to　NO2　bat　insensitive　to　other　disturbance　gases,　and　that　elements　W,　Sn　and　C　exist　in　SnO2/WO3/MWCNT　composite　thin　film　mainly　in　the　form　of　W+6,　Sn+4　and　C,　respectively.　Finally,　the　mechanism　of　gas-sensing　response　of　the　film　was　primarily　analyzed　and　discussed.