During the period from 1998 to 1999,absorption and emission of CO₂ in soil and vegetation system were studied by the static box method used in fields and biomass sampling,in the Lhasa Agro-ecology Station.This experiment was designed to explore carbon balance in the field ecosystem on the Qinghai-Tibet Plateau and its affecting factors,and disclose its contribution and response to the switch between pool and source of carbonaceous gas in the atmosphere.The CO₂ emissions in soil was more than the CO₂ assimilation of vegetation in the early wheat growing period(from sowing to jointing),while in the late period (from milking to maturity) the assimilation exceeded the emissions.The annual CO₂ emissions in soil were a little more than the annual assimilation of vegetation.