In the Sichuan Basin, where terrain is exceedingly complex, torrential rainfall occurs frequently, often leading to flash floods, debris flows and other disasters. When using mesoscale model to study torrential rainfall events over the complex terrain region, the horizontal grid size selection for mesoscale model is lacking in quantitative criteria. In order to reveal the complex terrain features in Sichuan and provide a quantitative basis for the horizontal grid resolution selection for mesoscale models, the two-dimensional discrete cosine transform is used to decompose terrain height fields and torrential rainfall distribution fields in Sichuan, China. Based on the torrential rainfall distribution, four study areas are selected to study spectral characteristics of terrain in the Sichuan Basin. Meanwhile, the issue of selecting the horizontal resolution for the numerical model is quantitatively discussed by using the terrain variance spectrum and numerical experiments. The major results show that: (1) the two-dimensional discrete cosine transform method can successfully demonstrate the anisotropic characteristics of the study areas; (2) terrain height variance and rainfall variance have better in-phase relationship in both the Ya'an region and the northwestern Sichuan Basin, while in the northeastern and central Sichuan Basin, out of phase relationship exists between them at greater wavelengths; (3) concerned with terrain features of a study area, ratio of model-resolved terrain variance to total terrain variance can be used to select the appropriate horizontal grid size for mesoscale models.