Spatial distribution diversity is the latest measurement method in pedodiversity researches, meanwhile, the actual distribution pattern and spatial location of soil patches as well as grid size are the main factors affecting the evaluation accuracy of soil spatial distribution diversity. 13 groups of spatial distribution diversity index in different grids from large-size (25 km) to small-size (500 m) were calculated by a case study of Henan Province for the analysis of diversity index variation in different grid sizes, this study also explored the criterion of how to choose the best grid size in the soil spatial distribution diversity evaluation. The results showed that the best grid size was 3–1 km in soil spatial distribution diversity evaluation when the basic soil taxon was soil family, and it should be ensured that the selected grid size was smaller than, or nearly close to the smallest soil taxon size when setting the grid size.