Abstract: In the three axis test of sand, there is an important relationship between the evolution of pore structure and its mechanical properties and strength characteristics. This paper based on synchrotron radiation micro CT scanning, combined with three axial compression experiment of silty sand soil in Hexi area of Nanjing, using self-developed micro three axis compression observation control system online, get images of Nanjing fine sand saturated pores in the load under different confining pressure in the process of structure in real time. Three-dimensional reconstruction of scanned image by ImageJ and Avizo software, get the pore structure of space image of the sand sample in the loading process, and then simplified as pore network model, then porosity, pore shape and pore radius were used to analyze the pore structure evolution. The results show that:(1)the loose condition of Nanjing fine sand under pressure showed typical strain softening characteristics of low confining, with the increase of confining pressure began to show characteristics of strain softening-hardening; (2)in the moderate compact state, the Nanjing fine sand showed a strain softening-hardening characteristic during the triaxial compression test, that is, from the shear state to the dilatancy state. Therefore, the porosity can be used to express the relationship between the porosity of peak strength and residual strength.(3)the interconnection of small pores is the cause of the emergence of large pores, which led to the loose condition of Nanjing fine sand appeared quasi-steady state.