To investigate the effect of external fixator in posture fixation during the interstitial of ¹²⁵I seeds implantation for head and neck cancer under CT guidance. 20 cases of head and neck cancer patients with CT guided radioactive ¹²⁵I particle implantation were enrolled. 10 patients of group A for external fixator, the other 10 of group B used conventional bind belt. Spiral CT scanning was performed with 5 mm slices, and we chose the first and last serial CT images during the operation for image fusion with automatic registration software, then we could get 3D displacement errors. Patients in group A of X, Y, Z axis position deviation were 1.06 cm, 1.22 cm and 1.22 cm , respectively. As to group B, they were 8.76 cm (P = 0.041), 9.99 cm (P = 0.009) and 0.009 cm (P = 0.023), respectively. The external fixator could applied in the treatment of head and neck cancer during CT-guided ¹²⁵I seed implantation, which fixed well in the operation, more comfortable, improve the positioning accuracy and reduce the extra rescanning radiation doses.