Abstract: Beased on the method of Lagrangian finite difference, a finite difference format of unified strain-hardening elastoplastic model with strength parameters of c and φ is established. The constitutive unified elastoplastic model is imported into the software of FLAC3D using a dynamic-link library file developed by VC++. The unified elastoplastic finite difference method is used to simulate the true triaxial tests on soft clay. Based on the unified elasto plastic model, the true triaxial tests (with different confining pressure and intermediate principal stress ratio) are simulated in FLAC3D, and the measured data are compared with the results of simulation. The analytic results suggest that the state of mechanical behavior of soft clay under complex stress conditions can be described by the unified strength theory when B=0.5.