Abstract: In order to find out the effects of temperature and pressure on the hydration swelling characteristics of clay mineral,based on the hydration of Wyoming sodium bentonite and calcium bentonite which is influenced by different temperature and pressure,the lab investigation of the hydration expansion characteristics of two clay mineral were carried by new HTP-3A dilatometer,and the microscopic changes in hydration expansion was analyzed from water content in clay mineral,particle morphology,capillary force and inter-particle interactions.etc.The results show that,temperature and pressure have an significant effect on hydration expansion characteristics of clay mineral and follow some certain rules.The higher the temperature,the greater the swelling capacity is and the faster the initial expansion rate is.There are two aspects of the role of pressure on the clay mineral hydration expansion closely related with temperature.And higher the pressure,the smaller the initial expansion rate is.Besides,the changes of water content in clay mineral,particle morphology,capillary force and inter-particle interactions are the important factors that the clay mineral has strong water swelling characteristics.The adhesive force between the clay particles changed greatly while the water content in clay mineral increases.The clay mineral has strong uneven water swelling characteristic and big swelling pressure which is led by the uneven changes of particle morphology.