Saddle-node bifurcation point of power flow functions is important in voltage stability analysis. A method based on the adjoint system theory is proposed. When load demands exceed the limit， there exist extraneous solutions in adjoint power flow functions and Lyapunov function value of extraneous solutions is nonzero. If appropriate Lyapunov function value is given， solutions near the bifurcation points can be obtained. A linear interpolation function can approximate the extraneous solutions curve. Combined with Moore-Spence system， saddle-node bifurcation point can be calculated. The simulation results of several standard IEEE test systems validate the proposed method.