The invention relates to an NO2 sensor based on NiO nano-films growing on an electrode surface in situ and a preparation method thereof, and belongs to the technical field of gas sensors of first-dimensional metallic oxide semiconductor materials. The sensor comprises an electrode element, and the NiO films which uniformly grow on the surface of the electrode element in-] situ, wherein the NiO films are composed of NiO crystal particles which are of face-centered cubic phase crystal structure and are 20-30nm in diameter. The gas sensor is capable of obtaining the maximum sensitivity for NO2 when working at the temperature of 150 DEG C, and is short in response and recovery time, high in selectivity, and high in long-term stability; the shortages of complex preparation technical process andlow selectivity and long-term stability of a traditional metallic oxide semiconductor type gas sensor can be effectively overcome; and the NO2 gas sensor has a good development prospect.