Abstract: Solution of MHD Riemann problem has a signficant implication to the study of the evolutional tendency of disturbances and the:interaction between shocks in the interplanetary space.Based on the assumption that a MHD coplanar discontinuity can be decomposed into a combination of forward and reverse shocks of fast and slow modes,centrical simple waves,and contact surfaces,this paper presents a three-parameter iteration method in solving MHD coplanar Riemann problems,applies this method to implement the decomposition of pure normal velocity discontinuities,and examines preliminarily the applicability of the method..