The direct shear system dynamics simulation model was developed on the basis of the modified Mohr-Coulomb yield criterion by using ANSYS／LS-DYNA and a combination of Lagrange and smoothed particle hydrodynamics (SPH). The dynamics simulation experiments of direct shear on four kinds of soil have been carried out. The mathematic models of soil shear strength and shearing speed were established by regression analysis. The research on soil shear failure process and the influence law and mechanism of shearing speed on shear strength was done. The results showed that with shearing speed increasing, shear strength increased. Third-order relationship exits between shear strength and shearing speed with small shear modulus. Logarithm relationship exits between shear strength and shearing speed with large shear modulus. The effect of shear modulus on shear strength is large. The large shear strength is with large shear modulus.