Abstract: The hydraulic fill is a kind of artificial soil,having such characteristics as large void proportion,strong compressibility,low strength.The indoor test result shows that the microstructure of hydraulic fill has a great in-fluence on it’s mechanics property,so it is a kind of strong structured soil.Through microstructure test,the SEM pictures of compression test soil sample under different axial stress are analyzed,and the relation between the void microstructure parameter and axial stress is studied,it is believed that the destroy stress exists during compress distortion.The compress character is different when the stress is larger or less than the destroy stress.The micro-cosmic proof is that the microstructure parameter changes suddenly,because the micro-structure is destroyed dur-ing compress distortion.At last the compression transforms mechanism of structured hydraulic fill is analyzed in brief,the result shows that the compressive process of hydraulic fill is thus divided into three stages:(a) the com-pression of gas in void and migration of the moisture content;(b) mutual slipping of the pellet,and(c) failure of soil structure and the formation of new structure.