Abstract: This paper established a flowing rule of soft clay with the cusp catasterophe. Only when the shear stressin saturated soft clay is greater than the long一term strength,the soil obeys the ideal plastic rule. But when theshear stress is smaller than the long一term strength the deformation of the soil decays gradually under a constantshear stress. A“γ-τ-t”relation with the cusp catastrophe model is proposed which agrees with the test data.