Abstract: Xijipangwan of Ningxia province is the intensive landslide area caused by the famous Haiyuan earthquake. Its loess is chosen as the research object. The dynamic triaxial tests are carried out on the undisturbed loess and the remolded loess under different water contents and different stress states. This paper examines the changes of microscopic images before and after dynamic triaxial tests. It discusses the action mechanism of confining pressure and moisture on loess soil particles and explains their influence characteristics on dynamic behaviors of loess from the perspective of the microstructure. The results indicate as follows. The relationship curve of dynamic shear modulus and dynamic shear strain can be regressed by derivation of the H-D model. With the increase of water content or the decrease of confining pressure, the value of parameter a and b increase. It is founded that there are some differences between the influence of water to undisturbed samples and remolded samples. With the increase of water content, dynamic shear modulus decreases, damping ratio increases. The dynamic characteristic parameters of remolded samples are transformed quickly before they are slow. Those parameters of undisturbed samples change slowly before they are quick. Besides the influence of external conditions(such as moisture and confining pressure), the microscopic characteristics of soil samples(such as pore size distribution, particle size, inter-particle spacing, connection mode and density) have great influence on the macroscopic behaviors(such as dynamic shear modulus and damping ratio) of loess.