It is found that the recently accumulated loess in one site at Shuping village , Yongdeng country, Gansu province, has special engineering behaviors. It has larger water collapsibility but only smaller seismic subsidence in natural humidity. Based on the analysis of microstructure images by means of scanning electron microscopy and experimental results of its physical parameters, the micro-mechanism of causing the special engineering behaviors is revealed. Some suggestions for construction on this kind site soil are presented.