Adjustments of Ankle-Foot Orthosis (AFO) stiffness is commonly prescribed for people with neurologically impaired to improve walking. It is important to quantify AFO stiffness levels to provide consistent patient-specific settings. We propose the Ankle Assistive Device Stiffness (AADS) test method, a simple design jig using motion capture system and musculoskeletal analysis software (OpenSim). The collected marker trajectory data were imported to OpenSim to calculate AFO dorsiflexion angle using inverse kinematics. Then a static optimization algorithm was used to identify external forces from operators and the AFO torque best matching the experimentally collected ground reaction force data. Estimated AFO moments were compared within the operators’ trials and with theoretically calculated AFO moments to evaluate the accuracy of AADS tests. This Project will include the model of AFO and ADDS and the scaling, IK and SOP setup sample.