Abstract: After introducing the concept of additional internal pressure, it is shown that the general suction consists mainly of the capillary suction and the additional internal pressure,and the effective stress should be decomposed into two parts: one caused by the general suction, and the other caused by the gravity and other external loads. Based on these concepts, we obtained a relation between the general suction and the shear strength for the unsaturated soil with low water content, and consequently a satisfactory explanation to the important experimental phenomenon of the unsaturated soil that the shear strength increases nonlinearly with the increasing of the general suction.