Unloading is a common condition that causes damage to geotechnical materials.In order to investigate the strength characteristics of the coarse-grained soil under three-dimensional unloading in different cases of the coefficient of intermediate principal stress b, several three-dimensional equal unloading tests in which the spherical stress p decreases with constant b and constant deviatoric stress q (300 kPa) were performed on coarse-grained soil by using true triaxial apparatus.The strength characteristics of coarse-grained soil were analyzed and the applicability of the commonly used strength criterions was studied.The test result shows that under the condition of three dimensional equal unloading, the strength parameter of coarse-grained soil is related to the coefficient of intermediate principal stress b.When b equals to 0, the internal friction angle φ_b is the minimum and the failure stress ratio M_b is the maximum.The stress ratio at failure decreases with the increase of b and, as b increases the gradient decreases gradually.The internal friction angle increases with b for small b values while decreases with the increase of b for large values.This can be explained by the co-impact of the shearing in three directions.All of the three strength criterions, including the Matsuoka-Nakai criterion, the criterion expressed by stress invariants of coarse-grained soil and Lade-Duncan criterion, can response the law between φ_b and b.It is found that the Matsuoka-Nakai criterion is the closest to the test results for small b values while the strength criterion expressed by stress invariants of coarse-grained soil is the closest to the test results for large b values.It is also found that the corner function strength criterion of coarse-grained soil is consistent with the experimental results.