The mechanism of cerebrospinal fluid in mitigating closed head injuries caused by mild impacts was investigated by finite-element modeling. Three biomechanical models were constructed. In these models, cerebrospinal fluid was considered as a soft solid material, an inviscid fluid without intracranial pressure, and an inviscid fluid with normal intracranial pressure, respectively, while other conditions such as the finite-element mesh, the impact, and the boundary conditions were kept the same. The recently developed nearest nodes finite-element method was adopted to deal with large deformations in brain tissue. Results obtained from the numerical studies showed that cerebrospinal fluid was able to remarkably reduce the maximum peak strains, especially the shear strains induced by impacts and transmitted to the brain. Cerebrospinal fluid with intracranial pressure was able to further buffer relative oscillations between the skull and the brain.