Abstract: Supernumerary robotic finger (SRF) is a wearable robot that achieves motion enhancement by augmenting additional limbs for the human body, however the grasp augmentation effect is limited. To address this problem, a maximum graspable sphere solution method based on SRF and hand pose is investigated to enhance the grasping ability, and the SRF design is optimized by quantitative analysis. Firstly, the D-H (Denavit-Hartenberg) kinematic models of the human hand and SRF are constructed, the finger pose library of the human hand and SRF is obtained by the Monte Carlo method, and an analytical method to achieve stable semi-envelope grasping of the sphere is proposed based on this library. Then, the grasp augmentation ability of SRF is defined and quantified based on the radius of the maximum graspable sphere of SRF and human hand, and the rod lengths of SRF at 3 wearing positions are optimized based on this ability. Finally, the SRF prototypes are built based on the subject's hand size and the rod lengths at 3 wearing positions before and after optimization, with which the grasping experiments are carried out on spheres with radius of 2~12 cm. The experimental results show that the grasping abilities at the 3 wearing positions are enhanced by 42.4%, 38.5% and 7.91% respectively after rod length optimization, which proves the correctness and effectiveness of the solution method.