Abstract: The semi conductive HCl gas sensor based on WO₃ is prepared by dopping,and the characteristic of that is studied.The special structure WO₃ is produced from Na²WO₃ and strong hydrochloric acid by sol-gel method.It’s found by XRD,SEM and TEM that the structure is sheet and incompact and the mean particle is about 17nm.The sensitivity to HCl can be improved greatly by dopping Al₂O₃ but the stability and selectivity is poor.Although the sensitivity is reduced by dopping a little ZnO,the stability can be improved greatly.The sensitivity to NO₂ can be reduced by increasing the heating voltage.The sensitivity mechanism and characteristic of WO₃ is analyzed.