This paper analyzed the disadvantage of the impeller root generating greater moment existing in the structure of large type axial flow fan impeller. A new type of axial flow fan impeller was designed based on analyzing the connection structure of helicopter rotor and hub. The dual hinged structure automatically balanced and eliminated the main force, which greatly reduced the stress generated by impeller root moment and improved the reliability of structure. According to the dual hinged design, 9.75m diameter prototype was produced and tested. The result was stable and reliable.