Abstract: In order to simulate the subgrade deformation caused by the coupling effect of wetting and traffic loads, the traditional wetting deformation test method is improved, and a set of upgraded wetting-deformation equipments are developed. It is regarded that the wetting deformation is generated by the loss of matrix suction, and a constitutive relationship between lost matric suction and wetting deformation deduced. Finally, the wetting-deformation regularity and modulus are studied by using the improved device. The results show that the initial compaction state of laterite has significant effect on the deformation, and the process can be divided into three stages: initiation, acceleration and stability ones. Deformation of the specimen without wetting mainly occurs at the initiation stage, while the wetting deformation of the specimen mainly occurs at the acceleration stage.