Brain death, whose assessment is of great significance, is the irreversible loss of all the functions of the brain and brainstem. The traditional diagnostic methods mainly relies on complex, harmful or unstable test, including apnea test,textbf evoked potential test, etc. Functional near infrared spectroscopy (fNIRS) utilize the good scattering properties of blood corpuscle to NIR, has the ability to monitor cerebral hemodynamics noninvasively. To objectively evaluate the brain death diagnosis with fNIRS, we use our portable fNIRS oximeter to measure the physiological data of fifteen brain death patients and twenty-two patients under natural state. The varied fractional concentration of inspired oxygen (FIO2) were provided in different phase. We found that the ratio ofthe concentration changes in oxy-hemoglobin to deoxy-hemoglobin(Δ[HbO2]/Δ[Hb])in normal patients is significantly lower than brain death patients, and its restore oxygen change process in low-high-low paradigm is more remarkable. This resulting promotion indicates potential of fNIRS-measured hemodynamic index in diagnosing brain death.