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 experments show that the composite thin film gas sensor exhibits excellent gas-sensing,and that is sensitive to NO₂ bat insensitve 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.