The patho-mechanism of scoliosis in patients with syringomyelia (SM) secondary to Chiari malformation type I (CMI) remains unknown up till now. In small sample studies published recently, there was an observation that curve direction tended to be on the same side as the dominant side of tonsillar displacement or the deviated side of syrinx location. Herein, a large-sample study of patients with single thoracic scoliosis secondary to CMI and SM was performed to validate these findings. The clinical and radiographic data of patients with scoliosis secondary to SM and CMI were reviewed. And patients who met the inclusion criteria were included. Based on the imaging presentation, the asymmetry of tonsillar displacement, syrinx location and scoliosis were judged by quantitative method. Then the imaging association among asymmetrically displaced tonsil, eccentrically located syrinx and curve direction were further analyzed. Thirty-nine patients with single thoracic scoliosis secondary to CMI and SM were included. 80% subjects showed concordance between the deviated side of eccentrically located syrinx and the dominant side of asymmetrically displaced tonsil. In 86% patients with asymmetrically displaced tonsil, thoracic curve was convex to the dominant side of tonsillar displacement. In 68% of patients with eccentrically located syrinx, the convex side of thoracic curve was on the same side as the deviated side of syrinx location. Asymmetrically displaced tonsils and eccentrically located syrinx are common imaging presentations in patients with SM secondary to CMI. The thoracic scoliosis in these patients was usually convex not only to the dominant side of asymmetrically displaced tonsils, but also to the side of eccentrically located syrinx, indicating that the asymmetry of tonsillar displacement and syrinx location might be involved in the regulation of curve direction.