Abstract: Failures of Loess slopes have been a bottleneck of rural urbanization and ground transportation construction in loess area.They and their hazards have been a current research focus of in engineering geology.One of the bases to reveal loess slope failure mechanism is to examine loess stress and microstructure changed due to effects of shock(vibration)or dynamic loading.This paper is based on the GDS Lab System,uses the equivalent sine wave loading gradually under strain controlling.Dynamic triaxial tests on loess samples are carried out.It further uses the scanning electron microscope to observe the differences of microstructures before and after the test.It analyzes the reasons of decreasing in dynamic strength.The results show the following findings.There is an obvious phenomenon of stress relaxation during the cyclic loading test on Malan loess.The degree of stress relaxation decreases with the increasing of confining pressure and strain.The dynamic failure stress increases linearly with the increasing of confining pressure.The Mole-Coulomb failure criterion is used to get the dynamic strength.Comparing the static strength and the dynamic strength,the dynamic internal friction angle decreases significantly,while the dynamic cohesion decreases slightly.The internal reason of decreasing on strength is as follows.By the role of cycling dynamic loading,the macro-cementation structure damages.On the other hand,the relationship between particles contact way changes from the point to point contact to the surface to surface contact.Thereby,the strength of the Malan loess is reduced.The micro-structure type is also changed into a mosaic micropore macro-cementation structure.