Objective:To monitor the cerebral vascular blood flow parameters in the early stage of simulated acute exposure to high altitude hypoxia by transcranial color Doppler (TCCD), and to evaluate the change trend of cerebral hemodynamics and cerebrovascular reactivity.Methods:Sixty-four healthy volunteers were selected to observe the changes of peak systolic flow velocity(Vs), end diastolic flow velocity(Vd), mean flow velocity(Vm), resistance index (RI) and pulsatility index (PI) of middle cerebral artery (MCA) 30 minutes after they quickly entered the simulated altitude of 4 500 meters. Combined with breath holding test, breath holding index (BHI) was used to evaluate cerebrovascular reactivity (CVR), and subjects were divided into ≤30 years old group and >30 years old group, and the changes of CVR after hypoxia of the two groups were compared.Results:In the early stage of hypoxic environment, compared with baseline, SpO 2 decreased, heart rate increased, and blood flow velocity of middle cerebral artery(Vs, Vd, Vm) increased significantly, BHI showed a decreasing trend (all P<0.01). After hypoxia, the BHI rate of change in >30 years old was lower than that of the subjects ≤30 years old ( P<0.05). After breath holding, cerebral blood flow velocity increased significantly, PI and RI decreased significantly (all P<0.01). Conclusions:Cerebral blood flow is very sensitive to hypoxia. The application of TCCD technology can evaluate the trend of cerebral blood flow dynamics and cerebrovascular reserve capacity in real time and accurately, which is helpful to provide objective basis and research basis for the prevention and treatment of high altitude hypoxia.