We used near-infrared spectroscopy technology to monitor and assess the treatment effect of dehydrating agent in injured rat brain in real time style. We employed the brain edema model in rats resulting from Feeney's freefall damage, then treated with different doses of mannitol, and collected reduced scattering coefficient (μ's) and intracranial pressure (ICP) values after the injury and during the treatment. The results showed that brain edema happened 1 h after the injury in rats' brain tissue, peaked around 72 h after injury, and then began to decrease gradually. The reduced scattering coefficient and ICP values of the treatment group injected with mannitol all decreased after administration. Compared with the effect of low-dose mannitol treatment, that of high-dose mannitol treatment was much better. The duration of the plateau was longer and most experiments results declined significantly. From this we conclude that the reduced scattering coefficient and ICP are consistent with the trend changes, and the reduced scattering coefficient could be used as an indicator for monitoring cerebral edema.