Abstract: In order to accurately estimate and evaluate grassland ecosystem carbon/sink and provide a scientific basis for rational utilization of grassland resources, the author used the eddy covariance method to analyze the day Xilinhaote steppe ecosystem CO₂ flux changes, changes of CO₂ fluxes at different growth stages and the relationship between pasture and environmental factors. The results showed that the grassland ecosystem CO₂ flux had obvious diurnal variation, absorbed CO₂ during the day and had mainly nocturnal emission performance of respiratory CO₂, changes was relatively stable in the night than during the day. CO₂ flux had the consistent trend in cloudy and sunny days, but greater change in cloudy days than in sunny days. At different forage growth stages and under different weather backgrounds, diurnal variations of CO₂ fluxes are different. CO₂ flux was greater in sunny days than in cloudy days, but no matter sunny or cloudy, stage of turning green, yellow and withered were the source of atmospheric CO₂, and flowering period was atmospheric CO₂ sinks. Net absorption of CO₂ fluxes of flowering period was significantly higher than that of the early flowering period and wilting period, and CO₂ respiration fluxes of wilting was significantly higher than that of the initial growing period and flowering period. Therefore, CO₂ flux of different growth stages is closely related to PAR intensity, the average temperature , average surface temperature, 5 cm equally modest average wind speed and other environmental factors .