###### Abstract  
In the context of global warming, the frequent occurrence of drought has become one of the main reasons affecting the loss of gross primary productivity (GPP) of terrestrial ecosystems. Under the influence of human activities, the vegetation greening trend of the Loess Plateau increased significantly. Therefore, it is of great significance to study the response of GPP to drought in the Loess Plateau under the greening trend. Here, we comprehensively assessed the ability of vegetation indices (VIs) and solar-induced chlorophyll fluorescence (SIF) to capture GPP changes at different seasonal scales and during drought. Specifically, we utilized three vegetation indices: normalized difference vegetation index (NDVI), near-infrared reflectance vegetation (NIRv), and kernel NDVI index (kNDVI), and determined the drought period of the Loess Plateau in 2001 based on the standardized precipitation evapotranspiration index (SPEI) and the standardized soil moisture index (SSMI). Moreover, the anomalies of VIs and SIF during the drought period and the relationship with GPP anomalies were compared. The results showed that both SIF and VIs were able to capture changes during the drought period as well as in normal years. Overall, SIF captured drought changes better due to water and heat stress as well as GPP changes compared to VIs. Across different time scales, SIF showed the strongest relationship with GPP (meanR\({}^{2}\) = 0.85), followed by NIRv (meanR\({}^{2}\) = 0.84), NDVI (meanR\({}^{2}\) = 0.76), and kNDVI (meanR\({}^{2}\) = 0.74), suggesting that SIF is more sensitive to physiological changes in vegetation. Notably, kNDVI performed best in sparse vegetation (meanR\({}^{2}\) = 0.85). In capture during drought, NIRv and kNDVI performed better in less productive land classes; SIF showed superior capture as land use class productivity increased. In addition, GPP anomalies correlated better with kNDVI anomalies (meanR\({}^{2}\) = 0.50) than with other index anomalies. In the future, efforts to integrate the respective strengths of SIF, NIRv, and kNDVI will improve our understanding of GPP changes. GPP; VIs; SIF; greening; drought; spatial and temporal variability; capture; Loess Plateau 2020215339 can also have an impact on local/regional climate. In general, an increase in ET due to the greening of vegetation enhances the local water vapor cycle, leading to an increase in precipitation in downwind areas and a weak but significant downward trend in near-surface air temperatures [8]. There is increasing evidence that the greater atmospheric water demand with rising temperatures (i.e., radiative effects of CO2) may lead to an increased intensity and frequency of drought, which could notably affect vegetation growth and crop yields [9; 10; 11]. Drought is a persistent and abnormal shortage of rainfall. The World Meteorological Organization (WMO) classified drought according to the affected domain as meteorological, agricultural, hydrological, and socioeconomic [12]. Drought is one of the most prevalent natural disasters in the world and has the most severe and widespread impacts on terrestrial ecosystem GPP. Droughts have tremendous impacts on ecosystem composition, structure, and functioning, largely affecting GPP accumulation and weakening the carbon sink function of terrestrial ecosystems [13; 14; 15]. In addition to this, drought events are often associated with complex emergencies involving multiple and compound hazards (e.g., food shortages, economic crises, or human/livestock/crop diseases). The interaction between a number of such factors can affect exposure, vulnerability, and capacity to cope with the crisis. Therefore, it is important to study the response of GPP to drought under the greening trend for future agricultural security and ecological protection. Changes in gross primary productivity (GPP) profoundly affect food security and the stability of the carbon cycle.