Abstract: It is very problem that strength decreases and deformation heightened of loess with special structure under seasonal rainfall and evaporation. Uniaxial compressive strength test is used to obtain uniaxial compressive strength of loess by dry-wet cycle. The relationship among the compressive strength, water content and dry-wet circle number is discussed. The micromechanism of intensity change is analysised by SEM image. The results show that the compressive strength nonlinear increases with decreasing water content. The compressive strength of loess decreases with increasing number of cycles and finally reaches stable. The contact style of coarse particle was changed and the quantity of colloidal substances and humus was decreased when water content increases and dry-wet cycles increases. Meanwhile, the pore diameter was increases, and the percentages of big pore and medium pore. The change law of macroscopic strength was described by microstructure of microstructure.