Cerebral ischemia is one of the common diseases in neurosurgery, and a rapid and accurate method for cerebral ischemia detection is urgently needed. Transmissive terahertz time-domain spectroscopy is applied to detect cerebral ischemic tissue. In order to reduce the impact of individual differences, the degree of ischemia of brain tissue is studied by calculating the relative difference of the terahertz absorption coefficients between left and right brains. It is found that with the increase of ischemia time, the relative difference of absorption coefficient between left and right brains of fresh tissue first increases and then decreases, which is mainly caused by the increase of water content and the decrease of cell density in the ischemic area; meanwhile, the relative difference of absorption coefficient between left and right brains of paraffin-embedded cerebral ischemic tissue gradually decreases, which is mainly due to the gradual decrease of cell density in the ischemic region. The results show that the terahertz time-domain spectroscopy system can be used to detect the earliest 2 h cerebral ischemia, which provides an effective technical means for the early label free rapid diagnosis of cerebral ischemia.