A methodology to calculate the meshing tooth of giant magnetostrictive harmonic motor is proposed in this paper. The deformation curve of the flex spline of the giant magnetostrictive harmonic motor was investigated with Fourier function. A kinematic model of harmonic drive was established when the involute profile is selected as the flexible gear tooth shape and the circular spline is obtained following the envelope theory, thereby generating the kinematic simulation as well as the trajectory of the circular spline teeth with respect to the flex spline. Finally, the relationship between the cone angle of flex spline and the engagement profile is discussed; together with an improvement strategy-equal width shrinkage of circular spline.