Abstract: Using the open path eddy covariance technique, we measured and analyzed the CO2 flux characteristics related to climate variables in a Larix gmelinii forest ecosystem (hereinafter referred to as “the ecosystem”). The results showed that: 1) In the growing season, the ecosystem CO2 flux changes with a diurnal cycle. The system assimilates CO2 in daytime and releases CO2 at night. The assimilation reaches its peak between 12:30 and 13:30. The CO2 flux ranges from -1.09 to 0.11mg/(m2·s) during the day. In the non-growing season, the ecosystem shows a carbon source with the CO2 flux ranging from 0 to 0.3mg/(m2·s). 2) In the growing season, there is a logarithmic relationship between the CO2 flux and photosynthetic active radiation (R2=0.4861), and the carbon sequestration capacity increases with the enhancement of PAR; therefore, PAR is a factor affecting CO2 flux directly. In the non-growing season, such relationship is not significant. 3) The CO2 flux is well correlated with air temperature (ta) in growing seasons (R2 = 0.6272), and CO2 flux is reduced with the rising ta, suggesting that ta is a main limiting factor in the ecosystem. In the non-growing season from December to February, the change of air temperature has no significant effects on CO2 flux. 4) The influence of soil temperature (ts) and soil moisture content (RH) on CO2 flux is mainly reflected in ecosystem respiration(Re). The soil moisture content ranges between 62%-87% in the ecosystem, with an average of 84% in the growing season and 67% in the non-growing season. Soil moisture content is not the main factor limiting CO2 flux in the ecosystem. On condition that water is not the limiting factor, soil temperature plays the major role on regulating CO2 flux in the ecosystem. Our research shows that there exists an exponential relationship between CO2 flux and soil temperature (R2=0.2826 in growing season and 0.2223 in non-growing season). Within a certain range, the rise of soil temperature will accelerate the metabolism of plants and microorganisms, and thus enhance the respiration of forest ecosystem and promote the emission of CO2.