Objective To explore a modified method of cerebellar tonsillectomy combined with posterior fossa decompression via small-size craniotomy for Chiari- Ⅰ malformation associated with syringomyelia and to evaluate its clinical efficacy. Methods The clinical data of 29 Chiari- Ⅰ malformation patients associated with syringomyelia, who underwent modified cerebellar tonsillectomy combined with posterior fossa decompression via small-size craniotomy from January 2012 to January 2014, were analyzed retrospectively.Patients were prone with head and neck in the coaxial position and received surgical intervention including posterior fossa decompression via small-size craniotomy, opening the rear atlas arch, resection of the inner part of cervical canal of the cerebellar tonsil herniation while keeping the integrity of soft meninges, adhesiolysis of median aperture of the fourth ventricle, dissection of the suture of arachnoid and dural edge to avoid latrogenic dead space, and suture of the dura with autologous fascia. The cerebrospinal fluid release rate of the whole process was controlled. Results A total of 24 patients were followed up after operation while 5 patients were lost in follow-up. The clinical symptom of 23 patients was improved within one year after operation. The repression of medulla oblongata and posterior upper part of cervical cord was removed in MRI examination 6 months after operation. The lower edge of cerebellar tonsillar was up to the plane above the foramen magnum and the herniation was resolved. The syringomyelia was shortened or disappeared. Conclusion Modified cerebellar tonsillectomy combined with posterior fossa decompression via small-size craniotomy, as a microscopy neurosurgery, is an effective method for the treatment of Chiari- Ⅰ malformation associated with syringomyelia.