Terrestrial ecosystems are the second largest carbon pool in the world, and their carbon budget has always been a hot field in climate change research. Any slight change in the ecosystem will cause changes in the global carbon cycle and budget, and the study of fertilization effect of carbon dioxide (CO₂) is the forefront part of global change carbon cycle. The relationship between CO₂ and ecosystem is complex, and the effect of CO₂ on carbon cycle of terrestrial ecosystem is still unclear at present. Current studies normally use the gross primary productivity (GPP) and net primary productivity (NPP) to study the impacts of CO₂ fertilization effect on ecosystem carbon sequestration, even though they do not completely explore the autotrophic and heterotrophic consumption of ecosystems. The net ecosystem productivity (NEP) used to study the fertilization effect of CO₂ on ecosystem carbon sequestration is more accurate and more valuable, but the related research is still relatively insufficient, currently. In this study, based on temperature data, solar radiation data, and normalized difference vegetation index data from remote sensing, a light energy utilization model was used to simulate the carbon cycle in Gannan, China in 2019. Three key carbon cycle indicators, namely GPP, NPP, and NEP were selected to analyze the spatial and temporal variation characteristics of ecosystem carbon sequestration and CO₂ fertilization effect in Gannan. The results showed that:(1) the total amount of carbon sequestration by ecosystem in Gannan in 2019 was about 2611 tC. The seasonal characteristics of GPP, NPP and NEP in Gannan were obvious, and their values reached the highest in summer. From the spatial perspective, the GPP and NPP were characterized by high in the east and low in the west while NEP was characterized by high in the north and low in the south in Ganan. (2) CO₂ had positive fertilization effect on GPP and NPP, which increased by 14.4% and 14.3%, respectively; However, it had a negative feedback effect on NEP, reducing it by 0.3%, and the overall influence of CO₂ on NEP was characterized by high in the north and low in the south in Ganan. This study reveals that although the positive fertilization effect of CO₂ is obvious in promoting GPP and NPP, the effect of CO₂ on NEP, namely carbon sequestration, is very limited in Gannan. Therefore, we should not make generalizations when studying the effects of CO₂ fertilization. The impacts of ecological geography on it cannot be ignored. This study can provide a certain theoretical basis for revealing the dynamic mechanism of terrestrial ecosystem carbon cycle.