The invention discloses an NO2 gas sensor based on an NiO nanocrystal and a preparation method of the NO2 gas sensor, wherein the preparation method specifically comprises the steps: coating an insulating substrate printed with an electrode with an NiO nanocrystal solution, and uniformly forming a film to obtain a NiO nanocrystal thin film; next, carrying out heat treatment on the NiO nanocrystalthin film under the condition of applying a voltage to the electrode; and then, applying an ageing voltage to the electrode, and carrying out continuous ageing for 6-10 days to obtain the NO2 gas sensor. The thin film based on an NiO semiconductor resistance-type gas sensor is obtained by improving a preparation process of a key gas-sensitive layer, and the gas sensor is capable of realizing filmformation on a ceramic substrate at room temperature, simple in preparation process, low in cost, free of high-temperature (400 DEG C or higher) treatment, capable of rapidly detecting the concentration change of a target gas at a relatively low temperature and high in response speed and has a good application prospect in a semiconductor-type gas sensor.