Abstract: The buoyancy convective flows in rectangular cross-section cavity of porous media saturated with fluid and heated form below are studied.The finite difference methods are used to study the bifuraction structure based on the bifurcation theory.Some typical figures of streamline and isotherm are given.The model exchange mechanisms of convection are discussed.Numenical results indude critical Rayleigh numbers and critical aspect ratios for exchange of some lower modes.