A three-degree-of-freedom dynamic model of a two-dimensional wing with an external store is considered. The governing equations of the wing/store system are established on quasi-steady aerodynamics. The flutter of the wing with cubic nonlinear stiffness in incompressible flow is studied by using Hopf bifurcation theory and numerical simulation. Furthermore, the effects of the system parameters on the flutter critical velocity are discussed. The results show that there is a sub-critical Hopf bifurcation for the wing/store system and the flutter critical speed will decrease versus increasing of the linear stiffness coefficient.