Abstract: In order to obtain the characteristics of strength of field expansive soil including cracks caused by wetting-drying cycles, the specimens both with and without undergoing wetting-drying cycles are tested by means of the field large shear apparatus. When the expansive soil does not undergo wetting-drying cycles, the shear stress reaches the peak value quickly and then reduces gradually; the stress-strain relations show the obvious softening. When the expansive soil undergoes wetting-drying cycles, the peak shear stress is much less than the values of expansive soil and the characteristics of softening are less obvious compared with those of the expansive soil which does not undergo wetting-drying cycles. The shear strength of expansive soil undergone wetting-drying cycles decreases to some degree compared to that of undisturbed expansive soil, and the decreased degree of cohesion is much greater than that of friction angle. It is shown that crack propagation due to wetting-drying cycles result in the reduction of shear strength, and it is the major factor to lead to instability and damage of projects in expansive soil areas.