The development of strain of saturated sand is studied based on the dynamic triaxial test. Hmsteresis Loop of the Saturated Sand in the dynamic triaxial Test is studied form view of the theory analytics and stress state. The result of research shows that attenuation of stiffness and appearance of shear dilatation affect the shape of hysteresis loop greatly, these explain the phenomenon in the dynamic triaxial Test, then the mechanism of the saturated sand liquefaction is more clear than before.