The geometry and band structures as well as the density of states of Ti-doped nonstoichiometric W18O49 nanowire are studied by employing the ab-initio plane-wave ultra-soft pseudo potential technique based on the density functional theory. Meanwhile, the adsorption and NO2-sensing properties of the doped nanowire are analyzed by further calculating the adsorption energy, planar averaged charge density difference and atomic Mulliken charge population of the NO2/Ti-W18O49 nanowire adsorption system. The results reveal that Ti-doping modifies the electronic structure and then the gas sensitivity of W18O49 nanowire obviously. After Ti-doping, new electronic states are introduced and the band structure near Fermi level (EF) is changed obviously, resulting in the variation of the band gap and EF position and then the increase of electronic conductivity. The adsorbed NO2 molecule acts as a charge accepter to extract electrons from the conduction band of W18O49 nanowire, causing the gas-sensing response due to the conductivity change of the nanowire. NO2 adsorption on Ti-doped W18O49 nanowire can cause more electrons to transfer from nanowire to NO2 molecule than the case on pure W18O49 nanowire, theoretically suggesting the validity of Ti-doping that can improve the sensitivity of W18O49 nanowire. The population calculations on different gas molecules adsorbed on Ti-doped W18O49 nanowire further indicate the much good sensitivity and selectivity of the doped nanowire to NO2 gas.