Syrinx resolution has been associated with an increase in the size of the posterior subarachnoid space (pSAS) after foramen magnum decompression (FMD) for type I Chiari malformation (CM1). The present study investigated the influence of pSAS increase on syrinx resolution and symptom improvement after FMD. 32 patients with CM1 with syrinx were analyzed retrospectively. FMD was performed for the 24 patients with CM1 with syrinx. pSAS areas were measured on sagittal magnetic resonance images. Neurological symptoms were grouped into three clinical categories and scored. The rates of symptom improvement in the CM1 patients with syrinx after FMD was 19.7% ± 12.9%. The mean times to the improvement of neurological symptoms in CM1 patients with syrinx after FMD was 23.4 ± 50.2 months. There were no significant differences between the patients with and without improvement of syrinx after FMD with regard to the age, length of tonsillar herniation, BMI, and preoperative pSAS areas. The rate of increase in the pSAS areas was significantly higher in the group with syrinx improvement within 1 year (p < 0.0001). All patients with a > 50% rate of increase in the pSAS area showed syrinx improvement. Our results suggested that the increasing postoperative pSAS area accelerated the timing of syrinx resolution.