Abstract: A series of laboratory and electron microscope tests are carried out on undisturbed soil samples of typical clay stratum taken from Hefei Metro tunnel construction site, and the expansive characteristics of typical clay soil samples in Hefei area are studied. Results show that: the clay samples in Hefei area have weak expansibility; the expansive force data of typical clays in Hefei area are obtained; there is a negative exponential function relationship between the shear strength parameters and water content of the clays in Hefei area; there are fracture structural planes and compact structural planes in the two types of clay, and the differences of micro pores, micro cracks and clay particle arrangement are the reasons for their different expansion characteristics.