There are a few references on thunder at present, moreover they are all about thunder frequency spectrum at observation location. However, thunder frequency spectrum in observation location is a distorted signal because of noises and sound attenuation effects. In this paper, thunder signal from lightning discharge channels is obtained in Lanzhou of China. By using digital filter and Fourier transform, thunder frequency spectrum at the observation location is gained. Then, thunder frequency spectrum near lightning discharge channel is computed with the theory of sound propagation effects.