Abstract: Xiaozhongdian Basin is a NNW-directed fault depression basin which was shaped while the Qinghai-Tibet Plateau intensely upheaved since the end of the Pliocene epoch,where the lacustrine clay of the middle and late Pleistocene is well distributed from the Xiaozhongdian River section.The Yunnan-Tibet Railway in planning will pass through the Xiaozhongdian Basin.The depth of lacustrine clay is above ten meters,which is obviously fine and has texture layer from the outcrop of the Xiaozhongdian.The laboratory testing results indicate that the Xiaozhongdian lacustrine clay is a kind of clay with high dispersion,high liquid limit,high plastic limit and high porosity ratio,whose mechanical characters are high shear strength and low expansibility or middle expansibility,so its engineering property is better.The slope will be destroyed when it experiences alternations of wetting and drying.