The displacement of tooth and the deformation of flexspline of harmonic drive in transmission state were studied by finite element model with shell element. A finite element model of flexspline based on shell element was built with taper teeth on the flexspline, and the contact relationship was defined by contact element between the flexspline and wave generator. The deformation distribution of the flexspline and the deformed position of the teeth on the flexspline were obtained by nonlinear contact analysis in ANSYS environment under the transmission forces according to the loading distribution in the reference. The position of teeth tip and teeth root in front, middle and back plane along teeth length were given by the deformation results. The research shows that the positions and orientations of the teeth in the planes have quite different distributions, which may influence mesh property evidently. The deformation in transmission state in the paper is of importance in heavy loading harmonic drive design because there is no theory results for flexspline in transmission state.