Abstract: The shear strength index is very important in geotechnical calculations,how to use the shear strength indices under different test conditions in engineering practice concerns whether the project is secure and stable,this problem has long been a rather hot debate in engineering rield.From the fundamentals,this paper elaborates some specious concepts,presents a thesis that calculating the water and the earth pressure together is insignificant in engineering,and the effective self-weight stress is not equal to the effective stress,meanwhile,it generalizes a basic idea that is how to choose the shear strength index in engineering practice Hope to have significance to accurately understand the theory of soil mechanics and direct engineering practice