Flying insects can achieve remarked and robust flapping aerodynamic performance and flight stability in various environments partially owing to the passive mechanisms of their wings and bodies, but a careful analysis associated with flexible wing hinge for forward flight is missing. Here we develop a fluid-structure interaction (FSI) model that couples one-torsional-spring-based elastic wing-hinge dynamics and flapping aerodynamics to study the aerodynamics and flight stability of hawkmoth Manduca Sexta with flexible wing hinge in various flight velocities. The results show that the leading-edge vortex (LEV), the body vortex (BV) and their interactions are responsible for augmenting the vertical force production interactively in all flight velocities, enabling a 6.5% increase in fast flight; the elastic storage in concern with flexible wing hinge exhibits a J-curve, achieving high power efficiency in intermediate forward flight. It is verified that the realistic wing-hinge stiffness can lead to an optimal aerodynamic performance in terms of vertical force production and power cost; external disturbance-rejection based on flexible wing hinge is of high robustness in multiple directions independent of forward flight velocities. This study thus points to the importance and significance of flexible wing hinge in biomimetic designs for flapping micro aerial vehicles.