Microstructure and gas sensing properties of partly agglomerating flower-like rutile TiO2 obtained using a one-pot hydrothermal method was investigated. Test results exhibited an excellent sensitivity to H-2 gas and stable reproducibility. Further, we performed a first principles calculation of an established surface model to delve into the sensing mechanism. Adsorbed oxygen preferred to locate upon the Ti-5c, site, while the electronic structure of surface had rather weak changes after hydrogen adsorption. This simulation may pave a new path to further discover the sensing mechanism of rutile TiO2-based gas sensor.