The area of urban forests and green-land is expanding dramatically across China in order to face rapid urbanization. Urban green-land ecosystems with plantations as their main vegetation type, have the great potential to sequestrate atmospheric carbon. Continuous measurements of CO₂ flux were made using eddy covariance technique from December 2011 to November 2012 in a mixed forest in Beijing Olympic Forest Park to quantify the seasonal dynamics of net ecosystem CO₂ exchange (NEE) and its responses to environmental factors. Gross ecosystem productivity (GEP), ecosystem respiration (Re), and net ecosystem productivity (NEP =-NEE) showed strong seasonal pattern, with CO₂ uptake dominating during the growing season from April to November, and a respiratory release of CO₂ dominating during the non-growing season. The carbon flux was influenced by photosynthetically active radiation (PAR), water vapor pressure deficit (VPD) and air temperature (T_a). In growing season, daytime net ecosystem carbon exchange (NEE_day) increased with increasing PAR. The ecosystem quantum yield (α) and maximum photosynthesis (A_max) showed an apparent seasonal pattern, both peaking in July. VPD also affected NEE through its direct effect on photosynthesis. NEE increased with the increasing PAR up to a threshold of 1200 μmol·m^-2·s^-1, then decreased with increasing PAR above this threshold. GEP, Re and NEP were all influenced by T_a, but responded differently. Re increased exponentially with air temperature (T_a), with the temperature sensitivity (Q₁₀) being 2.5. GEP also increased with T_a. This differential response of GEP and Re determined the relationship between NEP and T_a. NEP decreased with increasing T_a when T_a < 10.0 ℃, but increased when T_a > 10.0 ℃. NEE_day increased with PAR. The ecosystem quantum yield (α) and maximum photosynthesis (A_max) showed an apparent seasonal pattern, both peaking in July with the value of 0.083 μmol CO₂/μmol PAR and 29.46 μmol·m^-2·s^-1, respectively, and reaching a minimum in November with the value of 0.017 μmol CO₂/μmol PAR and 4.16 μmol·m^-2·s^-1. The predicted annual totals of GEP, Re and NEP were 1192, 1028 and 164 g C/m², respectively. The present results could contribute to the carbon budget of urban ecosystems, and help make carbon-oriented management strategies for sustainable urban development under global climate change.