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Forests play an important role in terrestrial carbon cycles. The mechanism underlying carbon balance in temperate deciduous broad-leaved forests is not clear. In this study, net ecosystem exchange (NEE) and environmental factors, including air temperature ( T a ), soil temperature ( T ) were continually measured using eddy covariance techniques in 2019 in a deciduous broad-leaved forest in Songshan, Beijing. We analyzed the characteristics of NEE and its response to environmental factors. The results showed that, at diurnal scale, the monthly averaged NEE exhibited a "U" shape curve ( s ), photosynthetically active radiation (PAR), vapor pressure deficit (VPD), soil water content (SWC) and precipitation ( P ) were continually measured using eddy covariance techniques in 2019 in a deciduous broad-leaved forest in Songshan, Beijing. We analyzed the characteristics of NEE and its response to environmental factors. The results showed that, at diurnal scale, the monthly averaged NEE exhibited a "U" shape curve ( i.e ., being a carbon sink over daytime while being a carbon source during nighttime) over the growing season. During the non-growing season, NEE was positive ( i.e ., carbon source) at diurnal scale. At the seasonal scale, NEE exhibited a unimodal curve. The annual cumulative NEE was -111 g C·m -2 ·a -1 . Annual ecosystem respiration was 555 g C·m -2 ·a -1 , while gross ecosystem productivity was 666 g C·m -2 ·a -1 . Carbon sequestration peaked in June, while emission peaked in November. PAR was the dominant factor affecting daytime NEE (NEE d ). VPD was the main factor that indirectly affected daytime NEE d , with an optimal VPD value that maximizes daytime NEE around 1-1.5 kPa. Soil temperature was the main factor affecting nighttime NEE (NEE n ). SWC was a limiting factor for NEE n . Too high or too low SWC would inhibit NEE n , with an optimal SWC value of 0.28 m 3 ·m -3 .