Method to investigate the elastic deformation of a flexspline in harmonic driver was developed under assembly force from wave generator and transmission force. Shell element and tapered beam element were applied to model the flexspline cone and the teeth on the flexspline respectively. Forced displacement of the flexspline in assembly state was imposed by contact analysis between the flexspline and wave generator. According to experimental distribution of tangential forces in transmission state, applying forces at the teeth tips on the flexspline in mesh state, deformation distribution under transmission torque loading was obtained. Simulations of assembly state and transmission state were realized in ANSYS software. In three sections of tooth ring vertical to the axis, the deformation distributions of the flexspline under specified displacement in assembly state were compared with the theory results. Deformation between tip of tooth and neutral layer in assembly state and in transmission state were compared respectively. The deformation results agreed well with the theory results in assembly state, while difference was found in tangential displacement on tooth tips in transmission state, which may influence mesh property evidently under transmission loading.