Objective To investigate the effect of posterior fossa decompression with resection of tonsils （PFDRT） in patients with Chiari malformation type I （CM-I） and syringomyelia， and to explore the related factors affecting the prognosis of patients.Methods Eighty-six adult patients with CM-I and syringomyelia who had been treated with PFDRT in the Department of Neurosurgery of the First Affiliated Hospital of Zhengzhou University from January 2016 to February 2022 were selected， and their clinical features and imaging features before and after surgery as well as follow-up data were analyzed. The Chicago Chiari Outcome Scale was used for assessment of patients’ clinical prognosis. Univariate and multivariate logistic regression analyses were used to analyze related influencing factors for patient’s prognosis.Results A total of 72 （83.72%） patients were clinically cured. Syringomyelia completely disappeared in 12 （13.95%） patients and was improved in 79 （91.86%） patients. There were postoperative fever in 14 （16.28%） patients and suboccipital effusion in 5 （5.81%） patients. Significant changes in imaging indicators were observed after surgery （P <0.001）. Postoperative clinical symptoms of most patients were improved significantly compared with preoperative ones （P <0.05）. Multivariate logistic regression analysis showed that disease duration and cerebellar symptoms were risk factors for clinical cure. Poorer prognosis was observed in patients with longer disease duration， and also in patients with cerebellar symptoms.Conclusions PFDRT is an effective method for the treatment of CM-I with syringomyelia. Both long disease duration and cerebellar symptoms affect the prognosis of patients. Early treatment should be provided for patients with CM-I and syringomyelia who have clinical symptoms.