Abstract: The multiple-scattering cluster method(MSC) for disorder system has been formulated based on dynamic theory of photoelectron diffraction.We have studied the structure of solid solution K_xNa_(1-x)C1,and found that the calculation results of mixed micro-crystal model is agreement with experiment excellently.It is shown that MSC method is suitable, especially,for structure studies of small molecule adsorbate-metal substrate system.