Triaxial soaking test system is developed; and test samples could be soaked to different saturations. Microstructures of Q2 loess of different saturations are observed by environmental scanning electron microscope (ESEM) and their mechanical behaviors are investigated through unsaturated triaxial tests. ESEM tests show that inter-aggregate pores of Q2 loess mainly consist of those that induce moderate or slight hydro-collapse. Confining pressure held constant, ration of inter-aggregate pores of larger size decreases with saturation; while that of inter-aggregate pores of 10-20 ?m increases. Results of unsaturated triaxial test show that stress-strain curves of Q2 loess of different saturations are all slightly strain-softening, and could be divided into three phases: rapid increase of deviatoric stress with axial strain, slight increase and slight decrease of deviatoric stress to a constant with axial strain. Relation between microstructure and mechanical behavior of Q2 loess after soaking is analyzed. A microstructural parameter is introduced that relates microstructure and mechanical behavior. A constitutive model of Q2 loess is established preliminarily considering influence of microstructure.