Abstract: The current domestic analysis method for relative content of sedimentary clay minerals is based on the PRC Oil and Gas Industry Standards SY/T5163-1995 "The X?ray Diffraction (XRD) Analysis Method for Relative Content of Sedimentary Clay Minerals".The disadvantage of this analysis method is that when the separation of clay in sandstone, by crushing the samples, soaking to less than 2 μm in diameter, the plastic debris composed of illite and illite/montmorillonite mixing layer, chlorite and chlorite/smectite mixed layer will inevitably be dispersed into the clay composition which may mixed with clay minerals in sand interstitial materials and directly influence the composition analysis of the clay minerals in sand interstitial materials in the reservoir, resulting in lacking fidelity of relative content of various clay minerals and affecting precise evaluation of the reservoir sensitivity. This paper proposes a new method for such a relative content analysis of interstitial clay minerals, by which the plastic debris can be separated from the interstitial materials, thus avoiding the lacking fidelity phenomenon