In order to analyze the damping ratio of soil under seismic load in frequency domain, stochastic process theory was introduced into soil damping ratio analysis. A frequency domain analysis method based on Fourier transform was proposed, and the feasibility of this method was verified by experiment and numerical analysis. The results showed that the damping ratio of soil under seismic load was a frequency-dependent function obtained by the frequency domain analysis method of Fourier transform. Any kind of soil and seismic wave can get a "damping ratio-frequency spectrum". We suggest that the soil-damping ratio should be regarded as a parameter that changes dynamically with the seismic wave input process. Through this frequency-domain analysis method, the "damping ratio-frequency spectrum" of every soil and every seismic wave can be found, and the dynamic damping ratio parameters can be obtained. In order to provide a reference for the determination of soil damping ratio under random load.