Spatial distribution patterns of species diversity and their relationships with scale are of great significance to understand the mechanism of community species diversity formation. Here, a field survey was conducted to assess the variation in species diversity along spatial scales in a karst evergreen and deciduous broad-leaved mixed forest, Southwest China. Based on the dataset of woody plants (DBH ≥ 1) from a 25 hm² plot, the spatial distribution patterns of species abundance, species richness, and Shannon-Wiener, Simpson, and Pielou's evenness indices were analyzed at six spatial scales-5 m×5 m, 10 m×10 m, 20 m×20 m, 50 m×50 m, 100 m×100 m, and 250 m×250 m. The results showed that spatial distributions of species diversity indices were highly heterogeneous. The variance of species diversity indices changed with spatial scale following a unimodal pattern, i.e., with a maximum value at the 100 m×100 m scale. Variation coefficients of species diversity decreased linearly with increasing scale. The Shannon-Wiener, Simpson, and Pielou's evenness indices decreased sharply at the 5 m×5 m and 20 m×20 m scales. No significant positive relationship was found between species richness and abundance at scales over 50 m×50 m (P > 0.05). Our results suggest that the spatial pattern of species diversity in this evergreen and deciduous broad-leaved mixed karst forest is closely related to spatial scale. To better understand spatial variation of species diversity across spatial scales, more such studies in other similar forest ecosystems are needed.