Abstract: Based on the concept of additional internal pressure, We revealed that the general suction in unsaturated sand with low moisture content consists of the capillary suction and the additional internal pressure,and the effective stress is decomposed into two parts:one caused by the general suction, and the other caused by the gravity and other external loads. According to these concepts,we obtained a relation between the general suction and the shear strength for the unsaturated sand with low moisture content, and consequently a satisfactory explanation to the important experimental phenomenon of the unsaturated sand:the shear strength increases nonlinearly with the increase of the general suction, but the converse situation would happen when the general suction exceeds a special value.