Background: Our study aims to introduce a dynamic interval ratio method calculated using cervical hyperextension-flexion X-ray films. Secondarily, we aim to evaluate the relationship between the posterior atlanto-occipital interval ratio and cervical spondylotic myelopathy and explain the rationale. Methods: We reviewed 83 cases with visible cervical dynamic X-ray films in our hospital from February 2015 to December 2018. Cases were divided into 2 groups according to their diagnosis (with or without spondylotic myelopathy). Radiographic measurements included the shortest distance between the posterior arch of the atlas and the occipital bone and cervical range of motion, and demographic data such as gender, age, and body mass index were also extracted. The posterior atlanto-occipital interval ratio (distance at hyperextension position/distance at hyperflexion position) was determined using logistic regression analysis models between the 2 groups. Results: We included 40 cases in the disease group and 43 cases in the control group. The mean posterior atlanto-occipital interval ratio was 0.65 +/- 0.30 (mean +/- standard deviation) in the disease group and 0.30 +/- 0.28 in the control group, with a significant difference (P<0.01). There was no correlation between the posterior atlanto-occipital interval ratio and gender or body mass index. However, the interval ratio had strong correlations with age, cervical spondylotic myelopathy, and Japanese Orthopaedic Association scores. Age, posterior atlanto-occipital interval ratio, and interval distance at hyperextension in the disease group were higher than those of the control group. Contrastingly, range of motion, Japanese Orthopaedic Association scores, and interval distance at the disease group's hyperflexion position were lower than in the control group. In all cases, the risk of cervical spondylotic myelopathy in the T2 group (cases with middle posterior atlanto-occipital interval ratio, according to the tertiles) was 6 times more than the T1 group (cases with lower ratio), and the T3 group (cases with higher ratio) had a 26.4 times greater risk than the T1 group. Conclusions: Our results suggest that the posterior atlanto-occipital interval ratio is a simple and meaningful parameter that could provide prognostic value for the risk of cervical spondylotic myelopathy through the imaging examinations of the selected cases. Higher posterior atlanto-occipital interval ratios indicate a greater risk for cervical spondylotic myelopathy and cervical musculoskeletal dysfunction. A higher posterior atlanto-occipital interval ratio may manifest undetected posterior atlanto-occipital stiffness, which needs more pathological evidence in future studies.