In order to investigate the mechanical properties of Zhanjiang structured clay in the three-dimensional stress state and the influence of structure on its strength criterion, a series of undrained true triaxial tests of different intermediate principal stress ratios, b-values, with equal mean principal stress, p-values, and undrained plane strain tests was carried out. The results are as follows. When p is less than or greater than the yield stress of the structure, the q- curves are strain softening and mild strain hardening type in general. The q- curves of true triaxial tests with different b-values are similar in shape. As b-value increases, the large principal strain corresponding to the q peak point shows the decreasing trend. Under different b-values, the effective cohesion decreases with increasing b-values, while the effective friction angle increases with increasing b-values. The effective cohesion and friction angle for the plane strain compression tests are between the effective cohesion and friction angle corresponding to b=0.25 and b=0.50. Influenced by the structure of clay, the strength criterion on π plane is applicable to the Lade-Duncan criterion when p-values are less than the yield stress of structure, while it is more applicable to the generalized nonlinear strength theory based on Mises criterion and Lade-Duncan criterion when p-values are greater than the yield stress of structure. For plane strain tests, the b-values of Zhanjiang clay under plane strain loading and unloading conditions are between 0.18 and 0.29 at failure, and the strength at failure can be approximated by the generalized plane strain strength criterion based on the general Mises and Lade-Duncan plane strain strength criterion.