According to the fact that the undrained shear strength decreases in the field vane test,applying the cylindrical expansion theory and assuming that the saturated soft clay satisfies Tresca yield criterion,the disturbance degree function D is given on the basis of the sensibility of saturated soft clay.Considering that the undrained shear strength is the linear function of the disturbance degree D in plastic region,the elastic-plastic solution of the cylindrical expansion has been gained.