Abstract: Inundating or humidifying process can easily destroy the distinct metastable structure of loess and control its mechanical behavior. This paper analyzes the microstructure of Malan loess at Zhaojiaan landslide area. It uses grain size analysis, scanning electron microscope, and mercury intrusion porosimetry. The results demonstrate that as the loess depth increases from surface to deep, the cementation of clay mineral is increasing gradually, the trellis pores is compressed obviously, and micro-pore is unchanged nearly. Triaxial compression test on loess with different initial water content shows that the strength of loess at top and bottom location decreases with increasing in water content. Since the loess at bottom location has higher clay cementation, its strength is larger than that at top. The cohesion strength parameter c is much more sensitive to water than the internal frictional angle . Therefore the strength reduction with depth is caused primarily by the decreasing of c. Water content increasing of loess at the bottom due to rising of ground water level can cause the parts of clay mineral to soften, undermine the cementation between particles, and continuously destruct the microstructure of loess, which resulted in wriggles of Zhaojiaan landslide.