It is generally recognized that swelling/shrinkage deformation and accumulative deformation of compacted expansive soil on wetting-drying cycles are strongly influenced by the specimen’s dry density and the net stress applied. However, investigations on the coupling effects of dry density and vertical net stress on the swelling/shrinkage deformation and accumulative deformation are limited. Cyclical wetting-drying tests were conducted on unsaturated compacted GMZ bentonite specimens with three dry densities under five vertical net stresses. The influence of vertical stress on accumulative deformation was analyzed. Then, the critical vertical stress was proposed for specimens with different dry densities. The effect of dry density on accumulative deformation was studied and the critical dry density was determined for different vertical stresses. Based on these, the concept of critical swelling-shrinkage line is proposed in the e-p space. According to the relationship between the initial state of specimen and the critical swelling-shrinkage line, a state parameter is proposed for determination of the accumulative strain on wetting-drying cycles.