This paper deals with the deformation function of the flexspline under load condition, and its effect on the tooth profile. In order to investigate the deformation discipline under load condition, a kinematic simulation analysis model of ABAQUS for a harmonic drive with engagement output is established. With this model, the nodal displacements of the flexspline neutral layer under different loads are obtained in dynamic transmission process. Fourier function is adopted to fit the deformation function under loads. Based on the deformation function, the tooth profile of the circular spline is modified. The example indicates that with the modified tooth profile under load condition the conjugate region increases, and the deformation function varies little. The work of this paper lays the foundation for optimizing the tooth profile to prolong service life of harmonic drives.