Abstract: With the development of all kinds of large scale engineering projects,soil-sock mixture(S-RM) as a new geotechnical material was proposed.And the study on the physical and mechanical properties of S-RM is the research project that the rock & soil mechanics and engineering geologist together faces now.As the study of S-RM is still at primary stage,S-RM is not separated from the traditional classification system of geotechnical materials,and there is no systematic discuss on the conception and classification of S-RM.Based on the present classification system of geotechnical materials and the former research achievements,the paper discusses on the conception and the key problems of S-RM in detail,and points that S-RM is a kind of extreme inhomogeneous geotechnical materials which is formed in Quaternary period and composed of large rock block,fine grained soil,pores and with certain percentage of rock block.The maximum observable dimension(MOD),soil/rock threshold(S/RT),strength comparison between soil and rock and percentage of rock block are the four key factors in the conception of S-RM.