Abstract: In order to assess the urban heat island effect on the engineering properties of clayey soil，some laboratory tests have been done on moisture contents and dry densities of the clayey soil under different environmental temperatures, and some thermodynamic characteristics of clayey soil are revealed. The research results indicate that the temperature has complicated influence on the shear strength of clayey soil. When the moisture content of soil is more than 18 %, the shear strength of soil decreases with the increasing temperature, showing the thermal softening on strength; When the moisture content is less than 18 % and dry density is 1.67 g·cm-3, the temperature hardly affects the soil strength; However when the dry density is less than 1.67 g·cm-3, the soil strength increases with the temperature rising, showing the thermal hardening on strength. Finally, the mechanisms of the thermal softening and hardening of soil are analyzed.