The desert steppe is a widely distributed grassland type on the northern slopes of the Tianshan Mountains, and it plays an important role in regulating the carbon balance of the grassland ecosystem in this area. The aim of this study was to elucidate the characteristics of carbon flux in desert steppe ecosystems on the northern slopes of the Tianshan Mountains and to determine the main environmental factors that influence carbon flux. A micro-meteorological observation system and a LI-840 CO2/H2O infrared analyzer were used to obtain continuous observation data. The data were quantitatively analyzed to detect variations in carbon flux in the desert steppe and the factors that influence it. During the growing season, the net ecosystem carbon exchange of desert steppe vegetation on the northern slopes of the Tianshan Mountains showed an inverted “U” curve on a daily scale. On a seasonal scale, the vegetation was a carbon sink in June, July, and August; and a carbon source in September and October. The total carbon assimilation in the desert steppe from June to October was 15.50 g C·m-2, and the highest carbon assimilation was 23.03 g C·m-2 in August. The trend in the ecosystem respiration rate showed a “single-peak” curve on a daily scale. On a seasonal scale, carbon emissions showed a trend of increasing and then decreasing, with the highest carbon emissions of 128.42 g C·m-2 in July. The daily variation in carbon flux in the desert steppe ecosystem was not significant in October. Photosynthetically active radiation was the main factor influencing the daily net ecosystem carbon exchange, and the relationship between these two factors fitted a rectangular hyperbola model. The daily net ecosystem carbon exchange decreased with increasing photosynthetically active radiation. The relationship between ecosystem respiration and the 5 cm soil temperature fitted the Van’t Hoff model, with a temperature sensitivity coefficient Q10 of 1.69 and an optimum soil water content of 0.16 m3·m-3. Excessively high or low soil water contents inhibited ecosystem respiration in this area.