Abstract: Study on the structural strength of soil is significant to the understanding of the soil cracking process and preventing related engineering geological problems. This paper takes the Xiashu soil in Nanjing as the object and uses the Super-Mini-Penetration(SMP)test. It analyzes the evolution characteristics of the structural strength of specimens during the drying process. The results show that the SMP test method is a better way for the quantitative study of structural strength of soil during the drying process. This method has the advantages of simple operation, high precision and a wide range of moisture content. During the drying process, the structural strength of specimen section remains stable and its spatial difference is very small in the high moisture content range(w24.27%).In the relative low water content range(w24.27%),the spatial difference of soil structural strength become more evident. As the penetration depth increase, the structural strength decreases and gradually become stable. The soil penetration stress increases exponentially as the average moisture content decreases, which is mainly due to the increase of soil suction caused by the soil water evaporation and the enhancement of interaction force among soil particles. In addition, the decrease of pore ratio and the increase of the soil particle joint point due to the shrinkage deformation of soil will also contribute to enhance the structural strength of clayey soil.