Abstract: This study estimated size-fractionated phytoplankton’s net primary productivity (NPP) in the Chinese coastal area using satellite data over the past two decades (2002－2022). Subsequently, we analyzed their spatiotemporal variations and identified driving factors. The results indicate that both total NPP and size-fractionated NPP exhibit higher values in coastal areas compared to offshore regions. Coastal total NPP was dominated by micro-phytoplankton NPP (NPP_micro), whereas offshore regions are primarily characterized by contributions from nano- (NPP_nano) and pico-phytoplankton NPP (NPP_pico), particularly NPP_pico. Seasonal patterns show peak values for total NPP and size-fractionated NPP in coastal regions like the Bohai Sea and the Yangtze River estuary during summer, while the central Yellow Sea and the offshore East China Sea exhibit peak values in spring. Long-term trends in total NPP and size-fractionated NPP remain generally stable over the past two decades. Correlation analyses show positive associations between total NPP and NPP_micro with the diffuse attenuation coefficient, along with negative correlations with sea surface wind speed. Sea surface temperature (SST) exhibits negative effects on total NPP and NPP_micro in the offshore of the East China Sea but demonstrates positive effects in other regions. Conversely, NPP_nano and NPP_pico display negative correlations with the diffuse attenuation coefficient, and SST also exerts negative driving effects in the offshore of the East China Sea.