Cerebral edema, as an acute head injury, has been of great interest over decades in clinical neurosurgery research. Many patients exhibited extensor motor abnormalities in the acute phase because their cerebral lose control of brainstem and spinal cord. These phenomenon, which is called “decerebrate rigidity”, showed up spontaneously or when their bodies were stimulated by external forces. Neuro imaging methods provide a new perspective in cerebral edema with decerebrate rigidity. Functional near infrared spectroscopy (fNIRS), as a novel optical detection method, has been widely used in monitoring changes of cerebral cortex blood oxygen concentration. Here, we utilized a self-made fNIRS detecting instrument with 4 channels fixed in forehead to monitor cerebral hemodynamic changes of patients with cerebral edema. The instrument has three waveforms (735nm, 805nm and 850nm) to detect concentration changes of oxygenated hemoglobin (HbO, deoxy hemoglobin (Hb), and blood volume (HbT). During the monitoring period, we captured two typical decerebrate rigidity in a patient (caused by external stimulation). At the beginning of decerebrate rigidity, Hb and HbT increased rapidly along with HbO decreased. The stage of this change lasts about 3s. Then the three hemodynamic parameters keep stable for about 6s. Finally, the three hemodynamic parameters return to the initial stage slowly (about 10s). The change amplitude of decerebrate rigidity caused by external stimulation were significantly higher than spontaneous situation. The results indicate that decerebrate rigidity caused acute hypoxia and congestion in patients’ cerebral cortex.