The current mainstream traveling wave method such as time domain reflectometry can not locate and identify the local moisture defects of the secondary cable. In order to solve this problem, this paper presents a method of locating local moisture defects of secondary cables based on reflection coefficient spectrum, and analyzes the moisture characteristics in detail. Firstly, the basic principle of the proposed positioning method is elaborated by establishing the distributed parameter model, frequency domain reflection theory, reflection coefficient spectrum and windowed Fourier transform of the secondary cable. Secondly, the model of secondary cable dampness defect is established, short cable samples are made for dampness experiment, and the influence of different dampness severity of secondary cable on the change of cable capacitance is deeply explained. Finally, the local moisture defects are made on a 22m long secondary loop cable in the laboratory, and the feasibility of the proposed method is verified by the experiment of the secondary loop cable.