In order to investigate the effect of intermediate principal stress on strength of sands, a series of true triaxial compression tests of Shanghai silty sand under different intermediate principal stress coefficient are performed on flexible true triaxial apparatus. Then the Lode angle dependent shape function of Mohr-Coulomb's strength criterion is modified based on true triaxial tests results and the true triaxial test results of Shanghai fine sands are simulated. The good predictions show that the strength criterion based upon true triaxial tests can describe the strength characteristics of sand in true 3D state preferably.