The subtropical ecosystem in the East Asian monsoon region is a significant carbon sink and plays an indispensable role in global carbon sequestration. However, the response of regional vegetation productivity to climate variability across seasons and the dominant climatic factors remain unclear. In this study, we analyzed the spatiotemporal patterns of the gross primary productivity (GPP) in Zhejiang Province, China, a representative subtropical monsoon region in East Asia, based on Community Land Model 5.0 (CLM5.0) simulations. The results show that the CLM5.0-based GPP agrees well with in situ observations (R = 0.69), with higher accuracy (e.g., higher R, lower RMSE and MAE) than Moderate Resolution Imaging Spectroradiometer product (MODIS), Two-Leaf Light Use Efficiency model simulations (TL-LUE) and global OCO-2-based SIF product (GOSIF). We further examined the spatiotemporal dynamics of three climatic factors (temperature, precipitation, and shortwave radiation) and their seasonal impacts on GPP. GPP increased at a rate of 3.40 g center dot C center dot m-2 center dot year-1 between 1979 and 2018. Using the sliding window method, a turning point in the GPP time series was detected in 2005, due to an abrupt increase in precipitation. In terms of multiyear average spatial distribution, GPP variations are primarily driven by shortwave radiation and precipitation, with the former exerting a stronger influence. Notably, the seasonal perspective reveals a different pattern. The influence of temperature on GPP becomes more pronounced, especially in spring and winter. This study is expected to enhance our understanding of carbon cycling processes within the biosphere and ecosystems of subtropical regions with monsoon climate, providing constraints and support for the development of Earth system models.