Abstract： Objective To investigate the effects of hyperbaric oxygen (HBO) on cerebral hemodynamics and ambulatory electroencephalogram(AEEG) of patients with moderate and severe brain trauma and to analyze its therapeutic efficiency. Methods One hundred and forty-two patients with moderate and severe brain trauma were randomly divided into the conventional therapy group (71 cases) and the HBO plus conventional therapy group (71 cases). The conventional therapy group received surgical treatment and conventional drug treatment, while the HBO group received the same treatment as the conventional therapy group and plus 3 courses of HBO treatment, following stabilization of the illness. Thirty normal healthy people who received physical check-ups were used as control. All the patients received Transcranial Doppler (TCD)detection, before treatment and on the 10th and 30th day after treatment. Ambulatory electroencephalogram (AEEG) was detected and the scores of Glasgow coma scale (GCS) were evaluated before treatment and on the 30th day after treatment. The scores of Glasgow outcome scale (GOS) were again measured in a clinical followup, following 6 months of brain trauma. Results Before treatment, Vs, Vm, Vd, PI and RI of the middle cerebral artery (MCA) in all the patients of both the conventional therapy group and HBO plus conventional therapy group were significantly higher than those of the normal control group ( P ＜ 0. 05 ) and the abnormal rates of AEEG accounted for 94. 4％ and 95.8％ respectively. On the 10th day after treatment, Vs, Vm, PI and RI in the conventional therapy group decreased significantly (P ＜ 0. 05), but were still higher than those of the normal control group ( P ＜ 0. 05 ). Vs, Vm, PI and RIin the HBO plus conventional therapy group decreased more significantly, when compared with those of the conventional therapy group ( P ＜ 0. 05 ),however, Vs, Vm and PI were still higher than those of the normal control group ( P ＜ 0. 05 ). On the 30th day after treatment, Vs and Vm in the conventional therapy group were lower than those of the normal control group (P＜0. 05), but, PI and RI were still higher than those of the normal control group (P ＜0.05). No significant differences in Vs, Vm, Vd, PI and RIcould be noted between the HBO plus conventional therapy group and the normal control group (P ＞0. 05 ). On the 30th day after treatment, abnormal rates of AEEG of both groups decreased significantly than those before treatment ( P ＜ 0. 05 ), while scores of GCS increased significantly than those before treatment ( P ＜ 0. 05 ) and more significant differences could be noticed in the HBO plus conventional therapy group, when compared with those of the conventional therapy group (P ＜ 0. 05). Six months after brain trauma, GOS scores indicated that the number of patients with good prognosis in the HBO plus conventional therapy group increased, and mortality rate decreased obviously, when compared with those of the conventional therapy group (P ＜ 0. 05 ). Conclusions HBO therapy could alleviate cerebral vascular spasms, regulate cerebral blood flow and promote the electrophysiological activity and function of neurons, thus enhancing therapeutic efficiency of the patients with moderate and severe traumatic brain injury.