In order to study the laws of the change of soil mechanical properties with temperature, the soil collected in the frozen soil area of the Tibet Plateau was re-prepared and subjected to different temperatures (15 ℃, 5 ℃, -5 ℃, -15 ℃) for direct shear test through an improved large-scale temperature-controlled direct shear equipment-FAST-1 under the condition of accurately controlling temperature changes and applying a large normal pressure. The temperature, shear strength, dilatancy effect, cohesion and internal friction angle were tested in the process of shearing. The test results show that the mechanical properties of silty clay can be changed greatly with the change of temperature. The shear strength of silty clay increases with the decrease of temperature under negative temperature condition, while the shear strength of silty clay remains almost unchanged under positive temperature condition. The maximum cohesion of silty clay under negative temperature is 1.8 MPa at -15 ℃, which is about 900 times of that under positive temperature. The internal friction angle shows a tendency of large change at both ends temperature and small change in the middle temperature. Due to the influence of temperature on cohesion and internal friction angle, the dilatancy of soil under negative temperature is obviously larger than positive temperature, up to 2.5mm, which is about 5 times of that under positive temperature.