This paper deals with the application of ultra-high density resistivity method to the detection of buried fault in the dam area of a reservoir, and describes geological situation of the survey area and the layout of surveying line as well as the detection mechanism of the method. On such a basis, apparent resistivity response of layered medium models is simulated by Hankel transform method. Based on inversion calculation combined with the geological drilling data, the buried depth of bedrock and the trend of regional fault are determined.The fault fracture zone presents obvious vein-like low resistivity anomalies in the middle of resistivity sections, which is consistent with the geological and drill hole data, thus verifying the effectiveness of the ultra-high density resistivity method.