Abstract: The performance of downscaling simulations from the Sixth International Coupling Model Comparison Program (CMIP6) was utilized to analyze the effects of human activities on the Seasonality Index (SI) across different climate zones in China. Moreover, possible future changes in rainfall seasonality due to various forcings under the medium emission scenario (SSP2-4.5) were examined. The results demonstrate a high consistency between the distribution of rainy season predictions of the downscaling CMIP6 model and the observed SI in China. The ensemble average of the downscaling simulations accurately captured the trends related to the onset and duration of the rainy season. In recent decades, human activities and natural forcings (ALL) have led to reduced precipitation yet increased rain season durations in China compared with scenarios with only natural forcing (NAT). However, the onset of the rainy season showed minimal differences between the NAT and ALL forcing scenarios. Changes in rainfall seasonality over the past 50 years are primarily attributed to alterations in the duration and intensity of the rainy season. Analyzing these impacts should provide scientific support for understanding variations in seasonal precipitation and formulating effective coping strategies.