Abstract: In order to obtain the strength evolution of coarse-grained soils under three-dimensional stress conditions, a series of true triaxial tests under different intermediate principal stress ratios (also called b value) are conducted for a kind of sandstone coarse-grained soil using the large-scale low friction true triaxial apparatus developed by Changjiang River Scientific Research Institute. The results show that: (1) The effect of the intermediate principal stress (σ₂) on strength index is significant. With the increase of b value, the strength of coarse-grained soils increases, especially when the b value increases from 0 to 0.25, compared with that of the conventional triaxial tests, the strength shows a significant growth. (2) Comparison of several classical strength criteria for soils indicates that the Lade-Duncan strength criterion is more suitable in revealing the strength evolution of the research samples. (3) A group of plane strain tests are also carried out. Under the confining stress conditions, when the deviatoric stress approaches the peak value, the values of b are stable at the range of 0.17 to 0.19, so, for practical projects, if the test condition is limited, the plane strain tests can be used to make a rough estimation for the strength index of coarse-grained soils under complex three-dimensional stress conditions. The conclusions will offer the references for determining the strength index and making a refine research on strength criteria of coarse-grained soils under complex stress conditions.