Abstract: Objective The three dimensional arterial spin labeling (3D-ASL) was used to study cerebral blood flow(CBF) in patients with severe obstructive sleep apnea syndrome (OSAS), and analyze the correlation of white matter perfusion abnormalities with apnea hypopnea index (AHI) and cognitive impairment. Methods Fifty-six cases of OSAS patients (AHI>30 times per hour) which were confirmed by clinic were included in this study. The control group had been set-up in age-matched 56 healthy volunteers without OSAS disease (AHI<5 times per hour). 3D-ASL sequences were examined in two groups of subjects using GE Discovery 750 MR. Cerebral blood flow (CBF) were measured in brain white matter (bilateral white matter of frontal lobe, parietal lobe white matter, bilateral anterior limb of the internal capsule, bilateral posterior limb of the internal capsule, bilateral external capsule, bilateral centrum semiovale, bilateral occipital lobe, bilateral temporal white matter and the genu of corpus callosum, corpus callosum and splenium of corpus callosum). Independent samples t test was used for statistical analysis of the two groups of the regional CBF. P values less than 0.05 were considered significant. The correlation between CBF value and AHI and MoCA score in the low perfusion areas was analyzed. Results CBF (mL/min·100 g) of the white matter area of severe OSAS group (the right frontal lobe white matter, left frontal lobe white matter, right parietal lobe white matter, left parietal lobe white matter, right occipital white matter and left occipital white matter, posterior branch of right and left internal capsule were 21.420±4.789, 21.787±4.713, 46.151±8.217, 47.112±7.800, 17.805±5.788, 17.356±5.365, 31.443±6.020, 30.636±5.890, respectively) were all decreased compared with the healthy control group (all P<0.05). The CBF value in the cerebral hypoperfusion areas were negatively correlated with the AHI value, and positively correlated with MoCA score. Conclusion White matter hypoperfusion areas could be found and measured in the severe OSAS patients by 3D-ASL sensitively, which correlated with the decline of cognitive function and apnea hypopnea index (AHI).