Abstract: Bifurcations of 2-multiple semi-stable limit cycles, as well as supercritical and subcritical Hopf bifurcations of an airfoil flutter system with cubic nonlinearity in incompressible flows were studied. Air speed and the linear stiffness coefficient of pitching were taken to form a 2-dimensional parameter plane, and the analytic solutions of critical boundaries of Hopf bifurcations were obtained in the 2-dimensional parameter plane. As a result, the critical speed and linear stiffness for bifurcations of the 2-multiple semi- stable limit cycles were determined by means of harmonic balance method.