N-type metal oxide semiconductors are widely used in the field of gas sensors, but achieving high sensor response at low temperatures with low concentrations is challenging. In this paper, we report a high-performance gas sensor for NO₂. SnO₂ grains with different sizes and oxygen vacancies were prepared by a simple hydrothermal method followed by high-temperature calcination, and the electronic and chemical properties of the SnO₂ grains were significantly changed. The results demonstrate that the sensor prepared from SnO₂ grains hydrothermally grown at 180 °C has the optimal gas sensing performance with a response up to 4358.8 for 1 ppm NO₂ at a low temperature of 80 °C, a rapid response recovery (7 s), and a detection limit down to 9 ppb. Together with some series characterization, an empirical factor F between the sensor response and the influencing factors is proposed. In addition, through experimental and density generalization studies, the oxygen vacancies are distinguished into surface oxygen vacancies and subsurface oxygen vacancies. The dominance of the surface oxygen vacancies in dictating the gas response is demonstrated. The excellent sensor characteristics demonstrated and the elucidation of the gas-sensitive mechanism provide a clear framework for the preparation of high-performance gas sensors.