Vortex spinning is a novel technology which produces short-staple yarns by utilizing high-speed swirling airflow. The structure of the spindle plays an important role in vortex spinning in terms of its effect on the resulting yarn properties. In this paper, a two-dimensional fluid-structure interaction (FSI) model for the fiber/air two-phase flow is presented to design the two spindle parameters—the spindle cone angle and spindle diameter by evaluating their effects on the fiber dynamics in the flow field inside the twisting system and the resulting yarn tenacity. The coupling between the fiber and airflow is solved and the motional characteristics of the fiber are obtained. It is found that the fiber moves downstream in a varying wavy shape and its spreaded trailing portion is then in a helical motion to form the yarn. The results also show that the increase of the spindle cone angle has a negative effect on the tenacity of the produced vortex yarn. The increased spindle diameter gives rise to the decreased vortex yarn tenacity. The numerical results can provide an explanation for the experimental results reported by previous studies.