The rapid development of urban rail transit has brought more and more serious harm to stray current, among which the rail-to-ground transition resistance is the most important factor affecting the size and distribution of stray current, and is the key to study stray current. At present, there is no effective method to detect rail-ground transition resistance. Therefore, by establishing a simulation model of subway rail, this paper studies the influence rules of different factors such as rail body resistance, soil resistivity and rail-ground transition resistance on the spectrum response characteristic curve of rail impedance, and compares and excludes the influence of other factors. It is concluded that the frequency range of the spectral response influenced by the rail-ground transition resistance is 10khz-50khz. And by using the frequency range, the key research rail transition resistance position and size of global and local factors change's influence on the impedance spectrum characteristic, put forward a kind of changing the nuances of the amplification effects of mathematical methods, namely to extract the effective spectrum characteristics, various factors to determine the rail line to make sure those goals are met, transition resistance whole such as substandard housing, The location where the local rail-ground transition resistance is not up to standard is located and the method is verified by field measurement. In this paper, a method based on frequency analysis is proposed to detect the rail to ground transition resistance, which is of great significance to the evaluation and prevention of stray current.