The ankle joint provides the largest joint torque during human lower limb motions. Therefore, ankle exoskeletons have received major attention in the research of lower limb augmented exoskeletons. Walking of a human equipped with an exoskeleton is a typical dynamics problem, while the research on human-exoskeleton coupling dynamics is still at an early stage. Concentrated on the cable-driven ankle exoskeleton, this paper developed a human-machine coupled dynamic model considering foot-ground interaction forces, human joint torques, and exoskeleton torques, by integrating the robot forward kinematics method and the Lagrange's equation, where the foot-ground interaction force was described by the Kelvin-Voigt model together with the Coulomb’s dry friction model, the human joint torque was generated by the PD control with the particle swarm optimization, and the assistive exoskeleton torque was determined by an upper-level controller in accordance with the human gait cycle. Through model-based dynamic simulations, this paper systematically analyzed the effect of the ankle exoskeleton assistance on human walking from the perspectives of the angle, torque, power, and work of the human ankle. It was demonstrated that when walking at a speed between 2.0 km/h and 6.5 km/h, human wearing the exoskeleton can achieve at least a 24.84% reduction in average ankle torque and at least a 24.69% reduction in ankle work. Musculoskeletal modeling and predictive simulations based on the SCONE were also performed in this paper. The simulation results showed that at a speed of 3.6km/h, wearing the exoskeleton can effectively reduce the peak level of soleus activation and the RMS value of the EMG signal by 6.21%, thereby validating the effect of the ankle exoskeleton assistance from a physiological perspective. Based on the results of this paper, the dynamic modeling and analysis method of human-exoskeleton coupled systems is further improved. The assistance mechanism of the ankle exoskeleton for walking is confirmed and interpreted from the perspectives of dynamics and physiology. This research also provides a theoretical basis for future experimental studies of lower-limb exoskeletons.