Carbon budgets for mature forests are important when studying carbon cycling in terrestrial ecosystems. To date, there has been little research on the carbon flux of a mature coniferous forest in the dark coniferous forest of Southwest China, and the role of the carbon cycle is not clear. In this study, the CO₂ flux of an Abies fabri mature forest was observed over a long period of time on Gongga Mountain in western Sichuan Province, China by using the eddy covariance technique. The data from June 2015 to May 2016 were used to analyze the variations in net ecosystem CO₂ exchange (NEE), ecosystem respiration (Re), and gross primary productivity (GPP) so that the carbon sources and sink conditions could be better understood. The data were combined with the environmental factors so that the main control factors affecting CO₂ flux could be analyzed. The results showed that (1) NEE had obvious diurnal variation characteristics and this produced a "U" shaped curve. The NEE was negative during the daytime, positive during the night, and reached its maximum around noon. The average daily NEE changes for each month were significantly different. The maximum peak for NEE appeared in June (-0.64 mgCO₂ m^-2 s^-1) and the minimum peak appeared in January (-0.08 mgCO₂ m^-2 s^-1). The earliest time when NEE changed from positive to negative occurred in winter and the latest in summer. (2) The NEE, Re, and GPP values showed obvious monthly variation characteristics. The NEE reached a maximum (-46.02 gC m^-2 month^-1) in June 2015 and a minimum (-1.42 gC m^-2 month^-1) in January 2016. The Re had a single peak variation, and the maximum and minimum values appeared in June 2015 (84.78 gC m^-2 month^-1) and January 2016 (12.82 gC m^-2 month^-1). The maximum and minimum GPP values appeared in June 2015 (130.81 gC m^-2 month^-1) and January 2016 (16.15 gC m^-2 month^-1), respectively. (3) Air temperature (T_a),5 cm soil temperature (T_s5), and photosynthetically active radiation (PAR) were the main environmental factors affecting the carbon fluxes in the Abies fabri mature forest. Carbon dioxide flux and air temperature had an exponential correlation (R²=0.5283, P < 0.01); CO₂ flux was significantly correlated with PAR during the daytime (R²=0.4373, P < 0.01); and CO₂ flux was significantly correlated with soil temperature (R²=0.4717, P < 0.01). (4) The annual NEE, Re and GPP values were -241.87, 564.81 gC m^-2and 806.68 gC m^-2, respectively. The results showed that mature Abies fabri forest on Gongga Mountain still has a strong carbon sequestration function.