Abstract: Previous micro-mechanism for clay deformation was focused on the compression. With the researches moving on, more attention has been paid to the micro-mechanism for clay rebounding. However, little relevant research has been done until now. The microscopic characteristics and their changes for both compression and rebound of the Shanghai clay are investigated in this paper through odometer tests, scanning electron microscope, and mercury intrusion porosimetry. The test results show that the change in micro-structure parameters can represent the macroscopic characteristics of clay deformation. When loaded, with the increasing consolidation pressure, the pore volume of clay decreases, the number of pores first increases and then decreases, the average pore shape coefficient increases first greatly and then slowly, and the pore shape fractal dimension first decreases greatly and then slowly. When unloaded, with the decreasing pressure, the number of pores increases, the average pore shape coefficient gently decreases, and pore shape fractal dimension slightly increases. In the compressing process, the percentages of medium and small pores change a little when the pressure is small, while the percentage of small pores increases greatly and that of medium pores decreases clearly when the pressure is great. In the rebounding process, the small pores are predominant under all levels of pressure, however, the percentage of medium pores is great for great pressure and it is small for small pressure. The rebound of clay results primarily from the increasing volume of small pores.