Abstract: The airfoil flutter is a self-excited aeroelastic phenomenon which may lead to catastrophe in aircraft flight. So its mechanism and rules are very important for wing and aircraft design. For a two degree-of-freedom airfoil model, the bifurcation behaviour of its limit-cycle flutter induced by the variation of the air flow velocity is investigated numerically by the Poincare map method. The Poincare section is defined in the sense so that it insects the orbit when the pitch angular acceleration crosses zero increasingly. The bifurcation diagrams due to the variation of the fluid force and the eight different typical phase portraits as well as the corresponding amplitude spectra are presented. It is found that the bifurcation sequence is formed by a series of super-harmonic bifurcations which possesses different directions and is also the reason why the eight phase portraits are topologically different.