Abstract: This paper is based on a comprehensive literature and a large number experimental tests. It comprehensively analyzes and studies the influences of main factors on strength of soft intercalation. The factors include substance, fabric characteristics, granularity composition, water content and ground stress. They affect the strength parameter of weak intercalation and are mutual restraint and influence, not isolated. Therefore, to take full consideration of all kinds of factors and to catch and hold the main controlling factors are the precondition that the strength parameter of weak intercalation is given correctly and reasonably. Natural characteristic index(w/wP) is a comprehensive index. It reflects the soil water content, degree of density, the types and contents of clay minerals, clay content and consistency conditions. This paper takes Xiangjiaba hydropower construction as an example. It studies the relativity between strength parameter and characteristic index(w/wP). It establishes a functional expression to describe the relation between strength parameter and characteristic index(w/wP). The expression is based on the results obtained from a large number of shear strength tests, water content and liquid/plastic limit determination test. These tests are performed at field or in laboratory. The results show that excellent correlation between strength parameter and characteristic index(w/wP). Therefore, use of relevant equations can conveniently give values of strength parameters of soft/weak intercalation when the natural characteristic index is known.