Abstract: This research studies the postural balance of a step stance through modeling, control, simulation and experimentation. The mathematical model of a five segment humanoid subject is developed. The redundant multi-variable system is projected onto a hybrid (Cartesian/joint) space with three degrees of freedom to make control easier. Based on the decoupled nonlinear system, a linear feedback control is designed to regulate the perturbation error. With the Lagrange method, a novel optimal bias control as a function of the system state, velocity and acceleration is designed to counteract the system main dynamics. The related balancing motion is performed by a human with markers on a force plate. The motion trajectories and the ground reaction forces are recorded. Comparison of the experimental and simulation results show that the proposed modeling and control strategy is capable of replicating the step stance balancing process. This research will promote understanding of mechanism of step stance postural balance for elder citizens and assist the relevant prosthesis design.