Abstract: The physical meaning of parameter X in the Muskingum method(M-method)is given by the hydraulics principle and the flood wave movement theory The storage equation of M-method is showed to be approximately the dynamic equation of difusion wave and the routing equation of M-method satisfied some conditions are proved to be two order accuracy solution of the diffusion wave equation Applicable condition and necessity of successive routing in subreaches of the M-method are suggested A equation of successive routing in subreaches of the M-method is derived by the Z transform method.