As the market demand for humanoid robots continues to grow, the requirements for the size and weight of harmonic reducers are gradually increasing. Compared with the CSG/CSF series, the CSD series has a shorter axial dimension, which is more conducive to further reducing the size and weight of the structure, and it is in line with the future direction of development. In this paper, based on a typical model of CSD-25-100, a parametric equivalent contact model of a short-cup harmonic reducer flexible gear was established, and the stresses of the short-cup flexible gear under the action of the wave generator/steel wheel was investigated. Firstly, the effects of the variations of key structural parameters of the flexible gear including the cylinder length, wall thickness, tooth width, and bottom fillet of the flexible gear, on the maximum equivalent force of the flexible gear was investigated; Then, a sensitivity study of the key parameters of the flexible gear is carried out by designing orthogonal experiments; Finally, the structural design of the short-cup harmonic drive is verified by numerical simulation to provide a basis for the non-standard structured design of the short-cup flexible gear.