Abstract. BACKGROUND:The 3D motion space of the human ankle is an important area of study in medicine. The 3D motion space can provide significant information for establishing more reasonable rehabilitation procedures and standards of ankle injury care. OBJECTIVE: This study aims to measure the 3D motion space of the human ankle and to use mathematical methods to quantify it. METHODS: A motion capturing system was used to simultaneously capture the 3D coordinates of points marked on the foot, and convert these coordinate values into rotation angles through trigonometric functions and vectors. The mathematical expression of the ankle's motion space was obtained by screening, arranging, and fitting the converted data. RESULTS: The mathematical expression of the 3D motion space of the participants was obtained. We statistically analyzed the data and learned that, in terms of 3D motion space, the right foot is more flexible than the left foot and the female foot is more flexible than the male foot. CONCLUSIONS: The adduction and abduction rotation ranges are affected by the plantar flexion or dorsal flexure rotation angles. This relationship can be expressed mathematically, which is significant in the study of the ankle joint.