In this study, based on kinematics data in steppage gait, a finite element model of human gait with ankle-foot orthosis (AFO) has been proposed to optimize the role of AFO through minimizing stress in the patients' sole. The required kinetics data for the model were captured through a force plate and then analysed by 3D-DOCTOR and ANSYS software. In the proposed three-dimensional finite element model the transmitted tension in soft tissue and bones during gait was calculated. By changing the thickness and materials of different layers of sole in AFO the tension variations have been assessed. Unlike previous studies, the effect of orthosis on tension generated in bones and muscles has been dynamically and continuously modelled and the contact between AFO and sole has been considered in this model. By using the optimized sole the stress distribution has been changed by +50.38% in the forefoot, +15% in the midfoot and -37.79% in the hindfoot. The tension reduction in the sole can improve the effect of AFO during abnormal gait. It is possible to design each orthosis sole based on the kinetics data of each patient.