Radiocarbon can be used to trace fossil fuel CO₂ (CO₂ff) in the atmosphere, because radiocarbon has been depleted in fossil fuels. Here we present our study on the spatial distribution and temporal variations of CO₂ff in Xi'an City, China using Δ¹⁴C of both green foxtail (Setaria viridis, L. Beauv.) leaf samples and urban air samples collected in the recent years. Our results show that the CO₂ff indicated by green foxtail ranged from 14.7 ± 1.7 to 52.6 ± 1.7 ppm, reflecting high CO₂ff mole fractions in downtown, industrial areas, and at road sites, and low CO₂ff mole fractions in public parks. Meanwhile, the monthly CO₂ff reflected by air samples showed higher value in winter (57.8 ± 17.1 ppm) than that in summer (20.2 ± 9.8 ppm) due to the enhancement usage of coal burning and the poor dispersion condition of atmosphere. This study displays that the increased fossil fuel emission is associated with the fast development of Xi'an City in China. It is worth mentioning that the green foxtail samples can be used to map out the CO₂ff spatial distribution on large scale quickly and conveniently, while the air samples can be used to trace the CO₂ff temporal variations with high resolution effectively. Therefore the Δ¹⁴C of both green foxtail and air samples is a good indicator of CO₂ff emission.