Optic neuritis is a common inflammatory disease of the optic nerve. And the cerebral spinal fluid (CSF) in the subarachnoid space of optic nerve is thought to be homogeneous as in spine. We report a case of optic neuritis, some unexpected opaque fluid observed to flow out from the optic canal during the optic canal decompression surgery when the CSF in spine is normal. One day after the surgery, the visual acuity of the patient improved dramatically to 0.4 from 0.05. This report highlights the possible restrictive pathological changes of the CSF in the optic nerve of acute optic neuritis, which may be the reason of the dysfunction of the optic nerve.