•The rainfall erosivity increased in the karst region of SW China from 1901 to 2020.•There are significant differences in the spatial distribution of annual, seasonal and monthly rainfall erosivity.•The trend of increasing rainfall erosivity is more obvious in January in winter.•More variability is found in the monthly scale study. Global warming changes global and regional patterns of precipitation, which will inevitably lead to more severe soil erosion risks in fragile karst ecosystems. Understanding the variation pattern of rainfall erosivity is vital for preventing and controlling soil erosion in the karst region of southwestern (SW) China. In this study, the spatiotemporal dynamic evolution pattern of the rainfall erosivity throughout the year in the karst region of SW China was explored using regression analysis and the Mann–Kendall test based on the monthly rainfall data for 1901–2020 in the CRU_TS 4.05 dataset. The results indicate that: (1) The rainfall erosivity in the karst region of SW China varies with latitude and is more significant in the south, and its interannual variations are linear and positive. (2) The interannual linear increase in rainfall erosivity was the greatest in the peak forest-plain rocky desertification control zone (FLPY). (3) The rainfall erosivity exhibited an increasing trend in spring, summer, and winter, with the most significant increase in January. (4) More variability was observed on the monthly scale. In summary, future investigations and control of soil erosion in the karst region of SW China under climate change should focus on the critical times when (e.g., January) and areas where (e.g., peak forest-plain rocky desertification control zone) rainfall erosivity changes significantly. Identifying the shift in the rainfall erosivity patterns caused by climate change is critical for assessing soil erosion risks in karst areas and for formulating countermeasures.