Unloading is a common condition that causes damage to geotechnical materials. In order to study 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 decreased with constant b and constant deviatoric stress q which was 300kPa were performed on coarse-grained soil by using the TSW-40 type true triaxial apparatus in Hohai University. The strength characteristics of coarse-grained soil under three-dimensional unloading condition were analyzed and the applicability of the commonly used strength criterions for the unloading coarse-grained soil 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; for different b values, when b equals to 0 the internal friction angle φb is the minimum and the failure stress ratio Mb 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 the increase of b for small b values while decreases with the increase of b for large b values which could be explained by the co-impact of the shearing in three directions; all of the three strength criterions which include Matsuoka-Nakai criterion, the strength criterion expressed by stress invariants of coarse-grained soil and Lade-Duncan criterion can response the law between φb and b; 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. The test result also shows that the corner function strength criterion of coarse-grained soil is consistent with the test results.